diff --git a/backend/static/css/04-dialogs.css b/backend/static/css/04-dialogs.css
new file mode 100644
index 0000000..9ad64a8
--- /dev/null
+++ b/backend/static/css/04-dialogs.css
@@ -0,0 +1,263 @@
+.warpbox-dialog-overlay {
+  position: fixed;
+  inset: 0;
+  z-index: 130;
+  display: grid;
+  place-items: center;
+  padding: 1rem;
+  background: color-mix(in srgb, var(--background) 60%, transparent);
+  backdrop-filter: blur(8px);
+  opacity: 0;
+  transition: opacity 160ms ease;
+}
+
+.warpbox-dialog-overlay.is-visible {
+  opacity: 1;
+}
+
+.warpbox-dialog {
+  position: relative;
+  width: min(28rem, 100%);
+  max-height: min(34rem, 90vh);
+  display: flex;
+  flex-direction: column;
+  overflow: hidden;
+  border: 1px solid var(--border);
+  border-radius: var(--radius);
+  background: var(--card);
+  color: var(--card-foreground);
+  box-shadow: var(--shadow);
+  opacity: 0;
+  transform: translateY(0.6rem) scale(0.98);
+  transition: opacity 160ms ease, transform 160ms ease;
+}
+
+.warpbox-dialog:focus {
+  outline: none;
+}
+
+.warpbox-dialog-overlay.is-visible .warpbox-dialog {
+  opacity: 1;
+  transform: translateY(0) scale(1);
+}
+
+.warpbox-dialog-head {
+  display: grid;
+  grid-template-columns: auto minmax(0, 1fr);
+  gap: 0.85rem;
+  align-items: center;
+  padding: 1.1rem 3.25rem 0 1.1rem;
+}
+
+.warpbox-dialog-icon {
+  width: 1.9rem;
+  height: 1.9rem;
+  display: grid;
+  place-items: center;
+  border-radius: 999px;
+  background: color-mix(in srgb, var(--primary) 20%, transparent);
+  color: var(--primary);
+  font-weight: 800;
+  line-height: 1;
+}
+
+.warpbox-dialog-warning .warpbox-dialog-icon {
+  background: color-mix(in srgb, var(--primary) 26%, transparent);
+  color: var(--primary-hover);
+}
+
+.warpbox-dialog-error .warpbox-dialog-icon {
+  background: color-mix(in srgb, var(--danger) 18%, transparent);
+  color: var(--danger);
+}
+
+.warpbox-dialog-title {
+  margin: 0;
+  font-size: 1.1rem;
+  line-height: 1.3;
+}
+
+.warpbox-dialog-close {
+  position: absolute;
+  top: 1.1rem;
+  right: 1.1rem;
+  z-index: 2;
+  min-height: 1.9rem;
+  height: 1.9rem;
+  width: 1.9rem;
+  padding: 0;
+  border-color: var(--border);
+  color: var(--muted-foreground);
+  background: var(--surface-1);
+  font-size: 1rem;
+  line-height: 1;
+}
+
+.warpbox-dialog-close:hover {
+  color: var(--foreground);
+  background: var(--surface-1-hover);
+}
+
+.warpbox-dialog-body {
+  padding: 0.85rem 1.1rem 1.1rem;
+  overflow: auto;
+}
+
+.warpbox-dialog-message {
+  margin: 0 0 0.75rem;
+  color: var(--muted-foreground);
+  font-size: 0.92rem;
+  line-height: 1.5;
+  overflow-wrap: anywhere;
+}
+
+.warpbox-dialog-message:last-child {
+  margin-bottom: 0;
+}
+
+.warpbox-dialog-field {
+  width: 100%;
+  border: 1px solid var(--input);
+  border-radius: calc(var(--radius) - 0.35rem);
+  background: var(--surface-1);
+  color: var(--foreground);
+  padding: 0.55rem 0.7rem;
+  font: inherit;
+}
+
+.warpbox-dialog-field:focus {
+  outline: 2px solid var(--ring);
+  outline-offset: 1px;
+}
+
+.warpbox-dialog-actions {
+  display: flex;
+  justify-content: flex-end;
+  gap: 0.55rem;
+  padding: 0 1.1rem 1.1rem;
+}
+
+html.warpbox-dialog-open,
+html.warpbox-dialog-open body {
+  overflow: hidden;
+  touch-action: none;
+}
+
+.dialog-file-list {
+  display: grid;
+  gap: 0.5rem;
+  margin-top: 0.25rem;
+  max-height: 14rem;
+  overflow: auto;
+  padding-right: 0.25rem;
+}
+
+.dialog-file-row {
+  display: grid;
+  grid-template-columns: auto minmax(0, 1fr) auto;
+  align-items: center;
+  gap: 0.65rem;
+  padding: 0.5rem 0.65rem;
+  border: 1px solid var(--border);
+  border-radius: calc(var(--radius) - 0.35rem);
+  background: var(--surface-1);
+}
+
+.dialog-file-icon {
+  width: 1.35rem;
+  height: 1.35rem;
+  color: var(--muted-foreground);
+}
+
+.dialog-file-name {
+  min-width: 0;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+  font-size: 0.86rem;
+}
+
+.dialog-file-size {
+  color: var(--muted-foreground);
+  font-size: 0.8rem;
+  white-space: nowrap;
+}
+
+:root[data-theme="retro"] .warpbox-dialog {
+  border: 1px solid #000000;
+  border-radius: 0;
+  background: #c0c0c0;
+  color: #000000;
+  box-shadow: inset -1px -1px 0 #404040, inset 1px 1px 0 #ffffff, inset -2px -2px 0 #808080, inset 2px 2px 0 #dfdfdf, 4px 4px 0 rgba(0, 0, 0, 0.45);
+  font-family: "PixeloidSans", "PixelOperator", "Microsoft Sans Serif", Tahoma, sans-serif;
+}
+
+:root[data-theme="retro"] .warpbox-dialog-head {
+  padding-top: 0.2rem;
+}
+
+:root[data-theme="retro"] .warpbox-dialog::before {
+  content: "Warpbox";
+  display: block;
+  margin: 0.18rem 0.18rem 0;
+  padding: 0.22rem 0.35rem;
+  background: linear-gradient(to right, #000078, 80%, #0f80cd);
+  color: #ffffff;
+  font-size: 0.78rem;
+  font-weight: 700;
+}
+
+:root[data-theme="retro"] .warpbox-dialog-error::before {
+  content: "Warpbox - Error";
+}
+
+:root[data-theme="retro"] .warpbox-dialog-warning::before {
+  content: "Warpbox - Warning";
+}
+
+:root[data-theme="retro"] .warpbox-dialog-info::before {
+  content: "Warpbox - Info";
+}
+
+:root[data-theme="retro"] .warpbox-dialog-icon {
+  border: 1px solid #000000;
+  background: #ffffff;
+  color: #000078;
+  box-shadow: inset 1px 1px 0 #808080, inset -1px -1px 0 #ffffff;
+}
+
+:root[data-theme="retro"] .warpbox-dialog-warning .warpbox-dialog-icon {
+  color: #9a5b00;
+}
+
+:root[data-theme="retro"] .warpbox-dialog-error .warpbox-dialog-icon {
+  color: #c00000;
+}
+
+:root[data-theme="retro"] .warpbox-dialog-message {
+  color: #000000;
+}
+
+:root[data-theme="retro"] .warpbox-dialog-close {
+  top: 0.36rem;
+  right: 0.3rem;
+  width: 1.1rem;
+  height: 0.95rem;
+  min-height: 0.95rem;
+  background: #c0c0c0;
+  color: #000000;
+  border: 1px solid #000000;
+  box-shadow: inset -1px -1px 0 #404040, inset 1px 1px 0 #ffffff, inset -2px -2px 0 #808080, inset 2px 2px 0 #dfdfdf;
+  font-size: 0.6rem;
+  font-weight: 700;
+}
+
+@media (max-width: 640px) {
+  .warpbox-dialog-overlay {
+    padding: 0.75rem;
+  }
+
+  .warpbox-dialog {
+    width: 100%;
+  }
+}
diff --git a/backend/static/js/04-dialogs.js b/backend/static/js/04-dialogs.js
new file mode 100644
index 0000000..7c0721a
--- /dev/null
+++ b/backend/static/js/04-dialogs.js
@@ -0,0 +1,299 @@
+(function () {
+  const VARIANTS = ["info", "warning", "error"];
+  const FOCUSABLE_SELECTOR = 'a[href], button:not([disabled]), textarea:not([disabled]), input:not([disabled]), select:not([disabled]), [tabindex]:not([tabindex="-1"])';
+
+  window.Warpbox = window.Warpbox || {};
+  let dialogIdCounter = 0;
+
+  function defaultTitle(variant) {
+    if (variant === "error") {
+      return "Error";
+    }
+    if (variant === "warning") {
+      return "Warning";
+    }
+    return "Info";
+  }
+
+  function normalizeOptions(options, message) {
+    if (typeof options === "string") {
+      options = { message: options };
+    } else {
+      options = options || {};
+    }
+    if (message) {
+      options.message = message;
+    }
+    const variant = VARIANTS.includes(options.variant) ? options.variant : "info";
+    return {
+      variant,
+      title: options.title || defaultTitle(variant),
+      message: options.message || "",
+      body: options.body || null,
+      actions: Array.isArray(options.actions) ? options.actions : [],
+      dismissible: options.dismissible !== false,
+      closable: options.closable !== false,
+      onClose: typeof options.onClose === "function" ? options.onClose : null,
+    };
+  }
+
+  function focusableElements(container) {
+    return Array.from(container.querySelectorAll(FOCUSABLE_SELECTOR)).filter((el) => el.offsetParent !== null);
+  }
+
+  function dialog(options, message) {
+    const config = normalizeOptions(options, message);
+    const previouslyFocused = document.activeElement;
+    dialogIdCounter += 1;
+    const titleId = "warpbox-dialog-title-" + dialogIdCounter;
+
+    const overlay = document.createElement("div");
+    overlay.className = "warpbox-dialog-overlay";
+
+    const card = document.createElement("div");
+    card.className = "warpbox-dialog warpbox-dialog-" + config.variant;
+    card.setAttribute("role", config.variant === "error" ? "alertdialog" : "dialog");
+    card.setAttribute("aria-modal", "true");
+    card.setAttribute("aria-labelledby", titleId);
+    card.setAttribute("tabindex", "-1");
+
+    const head = document.createElement("div");
+    head.className = "warpbox-dialog-head";
+
+    const icon = document.createElement("span");
+    icon.className = "warpbox-dialog-icon";
+    icon.setAttribute("aria-hidden", "true");
+    icon.textContent = config.variant === "error" ? "!" : config.variant === "warning" ? "?" : "i";
+
+    const title = document.createElement("h2");
+    title.id = titleId;
+    title.className = "warpbox-dialog-title";
+    title.textContent = config.title;
+
+    head.append(icon, title);
+
+    if (config.closable) {
+      const close = document.createElement("button");
+      close.type = "button";
+      close.className = "warpbox-dialog-close";
+      close.setAttribute("aria-label", "Close dialog");
+      close.textContent = "x";
+      close.addEventListener("click", () => closeDialog());
+      head.append(close);
+    }
+
+    const body = document.createElement("div");
+    body.className = "warpbox-dialog-body";
+
+    if (config.message) {
+      const text = document.createElement("p");
+      text.className = "warpbox-dialog-message";
+      text.textContent = config.message;
+      body.append(text);
+    }
+
+    if (config.body) {
+      const nodes = Array.isArray(config.body) ? config.body : [config.body];
+      nodes.forEach((node) => {
+        if (node instanceof Node) {
+          body.append(node);
+        }
+      });
+    }
+
+    card.append(head, body);
+
+    let autofocusTarget = null;
+    if (config.actions.length > 0) {
+      const actions = document.createElement("div");
+      actions.className = "warpbox-dialog-actions";
+      config.actions.forEach((action) => {
+        const button = document.createElement("button");
+        button.type = "button";
+        button.className = "button " + (action.kind === "primary" ? "button-primary" : action.kind === "ghost" ? "button-ghost" : "button-outline");
+        button.textContent = action.label || "OK";
+        button.addEventListener("click", () => {
+          if (typeof action.onClick === "function") {
+            action.onClick();
+          }
+          if (action.dismiss !== false) {
+            closeDialog();
+          }
+        });
+        if (action.autofocus) {
+          autofocusTarget = button;
+        }
+        actions.append(button);
+      });
+      card.append(actions);
+    }
+
+    overlay.append(card);
+    document.body.append(overlay);
+    document.documentElement.classList.add("warpbox-dialog-open");
+    window.requestAnimationFrame(() => {
+      overlay.classList.add("is-visible");
+      (autofocusTarget || card).focus();
+    });
+
+    function handleKeydown(event) {
+      if (event.key === "Escape") {
+        if (config.dismissible) {
+          event.preventDefault();
+          closeDialog();
+        }
+        return;
+      }
+      if (event.key !== "Tab") {
+        return;
+      }
+      const focusable = focusableElements(card);
+      if (focusable.length === 0) {
+        event.preventDefault();
+        return;
+      }
+      const first = focusable[0];
+      const last = focusable[focusable.length - 1];
+      if (event.shiftKey && document.activeElement === first) {
+        event.preventDefault();
+        last.focus();
+      } else if (!event.shiftKey && document.activeElement === last) {
+        event.preventDefault();
+        first.focus();
+      }
+    }
+
+    function handleOverlayClick(event) {
+      if (config.dismissible && event.target === overlay) {
+        closeDialog();
+      }
+    }
+
+    document.addEventListener("keydown", handleKeydown, true);
+    overlay.addEventListener("click", handleOverlayClick);
+
+    let closed = false;
+    function closeDialog() {
+      if (closed) {
+        return;
+      }
+      closed = true;
+      document.removeEventListener("keydown", handleKeydown, true);
+      overlay.removeEventListener("click", handleOverlayClick);
+      overlay.classList.remove("is-visible");
+      document.documentElement.classList.remove("warpbox-dialog-open");
+      window.setTimeout(() => overlay.remove(), 180);
+      if (previouslyFocused && typeof previouslyFocused.focus === "function") {
+        previouslyFocused.focus();
+      }
+      if (config.onClose) {
+        config.onClose();
+      }
+    }
+
+    return {
+      element: overlay,
+      close: closeDialog,
+    };
+  }
+
+  window.Warpbox.dialog = dialog;
+
+  window.Warpbox.alertDialog = function alertDialog(message, options) {
+    const config = (typeof options === "object" && options) || {};
+    return new Promise((resolve) => {
+      dialog({
+        ...config,
+        message: typeof message === "string" ? message : config.message,
+        actions: [{ label: config.okLabel || "OK", kind: "primary", autofocus: true }],
+        onClose: () => {
+          if (typeof config.onClose === "function") {
+            config.onClose();
+          }
+          resolve();
+        },
+      });
+    });
+  };
+
+  window.Warpbox.confirmDialog = function confirmDialog(message, options) {
+    const config = (typeof options === "object" && options) || {};
+    return new Promise((resolve) => {
+      let settled = false;
+      function settle(value) {
+        if (settled) {
+          return;
+        }
+        settled = true;
+        resolve(value);
+      }
+      dialog({
+        ...config,
+        message: typeof message === "string" ? message : config.message,
+        actions: [
+          { label: config.cancelLabel || "Cancel", kind: "outline", autofocus: true, onClick: () => settle(false) },
+          { label: config.confirmLabel || "Confirm", kind: "primary", onClick: () => settle(true) },
+        ],
+        onClose: () => {
+          if (typeof config.onClose === "function") {
+            config.onClose();
+          }
+          settle(false);
+        },
+      });
+    });
+  };
+
+  window.Warpbox.promptDialog = function promptDialog(message, options) {
+    const config = (typeof options === "object" && options) || {};
+    return new Promise((resolve) => {
+      let settled = false;
+      function settle(value) {
+        if (settled) {
+          return;
+        }
+        settled = true;
+        resolve(value);
+      }
+
+      const field = document.createElement("input");
+      field.type = config.inputType || "text";
+      field.className = "warpbox-dialog-field";
+      if (config.placeholder) {
+        field.placeholder = config.placeholder;
+      }
+      if (typeof config.value === "string") {
+        field.value = config.value;
+      }
+
+      let controller = null;
+      field.addEventListener("keydown", (event) => {
+        if (event.key === "Enter") {
+          event.preventDefault();
+          settle(field.value);
+          if (controller) {
+            controller.close();
+          }
+        }
+      });
+
+      controller = dialog({
+        ...config,
+        message: typeof message === "string" ? message : config.message,
+        body: field,
+        actions: [
+          { label: config.cancelLabel || "Cancel", kind: "outline", onClick: () => settle(null) },
+          { label: config.okLabel || "OK", kind: "primary", onClick: () => settle(field.value) },
+        ],
+        onClose: () => {
+          if (typeof config.onClose === "function") {
+            config.onClose();
+          }
+          settle(null);
+        },
+      });
+
+      window.requestAnimationFrame(() => field.focus());
+    });
+  };
+})();
diff --git a/backend/static/js/40-upload.js b/backend/static/js/40-upload.js
index a33ff12..4965d48 100644
--- a/backend/static/js/40-upload.js
+++ b/backend/static/js/40-upload.js
@@ -17,6 +17,7 @@
   const RESUMABLE_SESSIONS_KEY = "warpbox-resumable-sessions";
   const SHARE_CACHE = "warpbox-share-target-v1";
   const SHARE_LATEST_KEY = "/__warpbox_share_target__/latest";
+  const CELLULAR_WARNING_THRESHOLD_BYTES = 200 * 1024 * 1024;
 
   if (!form || !dropZone || !fileInput) {
     return;
@@ -106,6 +107,12 @@
     if (!validateSelectedFilesWithinLimit(selectedFiles)) {
       return;
     }
+    if (isSlowOrMeteredConnection() && totalSelectedBytes(selectedFiles) >= CELLULAR_WARNING_THRESHOLD_BYTES) {
+      const proceed = await confirmCellularUpload(selectedFiles);
+      if (!proceed) {
+        return;
+      }
+    }
 
     const submit = form.querySelector("button[type='submit']");
     const formData = uploadFormData();
@@ -228,6 +235,56 @@
     }
   }
 
+  function isSlowOrMeteredConnection() {
+    const connection = navigator.connection || navigator.mozConnection || navigator.webkitConnection;
+    if (!connection) {
+      return false;
+    }
+    if (connection.saveData === true) {
+      return true;
+    }
+    return ["slow-2g", "2g", "3g"].includes(connection.effectiveType);
+  }
+
+  function totalSelectedBytes(files) {
+    return files.reduce((sum, file) => sum + file.size, 0);
+  }
+
+  function confirmCellularUpload(files) {
+    const list = document.createElement("div");
+    list.className = "dialog-file-list";
+    files.forEach((file) => {
+      const icon = document.createElement("span");
+      icon.className = "svg-icon svg-icon-document dialog-file-icon";
+      icon.setAttribute("aria-hidden", "true");
+
+      const name = document.createElement("span");
+      name.className = "dialog-file-name";
+      name.textContent = file.name;
+      name.title = file.name;
+
+      const size = document.createElement("span");
+      size.className = "dialog-file-size";
+      size.textContent = window.Warpbox.formatBytes(file.size);
+
+      const row = document.createElement("div");
+      row.className = "dialog-file-row";
+      row.append(icon, name, size);
+      list.append(row);
+    });
+
+    const totalLabel = window.Warpbox.formatBytes(totalSelectedBytes(files));
+    const message = `You're on a slow or metered connection. You're about to upload ${files.length} file${files.length === 1 ? "" : "s"} (${totalLabel} total) — this could take a while or use up your data plan.`;
+
+    return window.Warpbox.confirmDialog(message, {
+      title: "Slow connection detected",
+      variant: "warning",
+      body: list,
+      confirmLabel: "Upload anyway",
+      cancelLabel: "Cancel",
+    });
+  }
+
   function isShareTargetLaunch() {
     const params = new URLSearchParams(window.location.search || "");
     return params.has("share-target");
diff --git a/backend/templates/layouts/base.html b/backend/templates/layouts/base.html
index 7d58ee5..3077dc8 100644
--- a/backend/templates/layouts/base.html
+++ b/backend/templates/layouts/base.html
@@ -54,6 +54,7 @@
 
   <script src="/static/js/05-theme.js?version={{.AppVersion}}"></script>
   <link rel="stylesheet" href="/static/css/00-base.css?version={{.AppVersion}}">
+  <link rel="stylesheet" href="/static/css/04-dialogs.css?version={{.AppVersion}}">
   <link rel="stylesheet" href="/static/css/10-layout.css?version={{.AppVersion}}">
   <link rel="stylesheet" href="/static/css/15-revamp.css?version={{.AppVersion}}">
   <link rel="stylesheet" href="/static/css/16-retro.css?version={{.AppVersion}}">
@@ -70,6 +71,7 @@
   <script defer src="/static/js/00-utils.js?version={{.AppVersion}}"></script>
   <script defer src="/static/js/02-pwa.js?version={{.AppVersion}}"></script>
   <script defer src="/static/js/03-popups.js?version={{.AppVersion}}"></script>
+  <script defer src="/static/js/04-dialogs.js?version={{.AppVersion}}"></script>
   <script defer src="/static/js/10-file-browser.js?version={{.AppVersion}}"></script>
   <script defer src="/static/js/12-reactions.js?version={{.AppVersion}}"></script>
   <script defer src="/static/js/13-share.js?version={{.AppVersion}}"></script>