feat(backend): handle processing errors and add PWA routes

- Block file downloads and previews with a 424 StatusFailedDependency if file processing failed or the box has issues.
- Register routes for `/service-worker.js` and `/share-target` to support PWA features.
- Update README.md with an AI usage disclosure.

backend/static/js/03-popups.js

@@ -0,0 +1,174 @@
+(function () {
+  const DEFAULT_DURATION = 6200;
+  const VARIANTS = ["info", "warning", "error"];
+  const GENERIC_ERROR_MESSAGE = "Something went wrong on this page. Please try again in a moment.";
+
+  window.Warpbox = window.Warpbox || {};
+  let lastGlobalErrorAt = 0;
+
+  function ensureRegion() {
+    let region = document.querySelector("[data-warpbox-popups]");
+    if (region) {
+      return region;
+    }
+    region = document.createElement("div");
+    region.className = "warpbox-popups";
+    region.setAttribute("data-warpbox-popups", "");
+    region.setAttribute("aria-live", "polite");
+    region.setAttribute("aria-atomic", "false");
+    document.body.append(region);
+    return region;
+  }
+
+  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 || "",
+      duration: Number.isFinite(options.duration) ? options.duration : DEFAULT_DURATION,
+      actions: Array.isArray(options.actions) ? options.actions : [],
+    };
+  }
+
+  function defaultTitle(variant) {
+    if (variant === "error") {
+      return "Error";
+    }
+    if (variant === "warning") {
+      return "Warning";
+    }
+    return "Info";
+  }
+
+  function notify(options, message) {
+    const config = normalizeOptions(options, message);
+    const region = ensureRegion();
+    const popup = document.createElement("section");
+    popup.className = "warpbox-popup warpbox-popup-" + config.variant;
+    popup.setAttribute("role", config.variant === "error" ? "alert" : "status");
+
+    const chrome = document.createElement("div");
+    chrome.className = "warpbox-popup-chrome";
+
+    const icon = document.createElement("span");
+    icon.className = "warpbox-popup-icon";
+    icon.setAttribute("aria-hidden", "true");
+    icon.textContent = config.variant === "error" ? "!" : config.variant === "warning" ? "?" : "i";
+
+    const body = document.createElement("div");
+    body.className = "warpbox-popup-body";
+
+    const title = document.createElement("strong");
+    title.className = "warpbox-popup-title";
+    title.textContent = config.title;
+
+    const text = document.createElement("p");
+    text.className = "warpbox-popup-message";
+    text.textContent = config.message;
+
+    body.append(title, text);
+
+    const close = document.createElement("button");
+    close.type = "button";
+    close.className = "warpbox-popup-close";
+    close.setAttribute("aria-label", "Dismiss notification");
+    close.textContent = "x";
+    close.addEventListener("click", () => dismiss(popup));
+
+    chrome.append(icon, body, close);
+    popup.append(chrome);
+
+    if (config.actions.length > 0) {
+      const actions = document.createElement("div");
+      actions.className = "warpbox-popup-actions";
+      config.actions.forEach((action) => {
+        const button = document.createElement("button");
+        button.type = "button";
+        button.className = "button " + (action.kind === "primary" ? "button-primary" : "button-outline");
+        button.textContent = action.label || "Action";
+        button.addEventListener("click", () => {
+          if (typeof action.onClick === "function") {
+            action.onClick();
+          }
+          if (action.dismiss !== false) {
+            dismiss(popup);
+          }
+        });
+        actions.append(button);
+      });
+      popup.append(actions);
+    }
+
+    region.append(popup);
+    window.requestAnimationFrame(() => popup.classList.add("is-visible"));
+
+    let timer = null;
+    if (config.duration > 0) {
+      timer = window.setTimeout(() => dismiss(popup), config.duration);
+    }
+
+    return {
+      element: popup,
+      close: function closePopup() {
+        if (timer) {
+          window.clearTimeout(timer);
+        }
+        dismiss(popup);
+      },
+    };
+  }
+
+  function dismiss(popup) {
+    if (!popup || popup.dataset.closing === "true") {
+      return;
+    }
+    popup.dataset.closing = "true";
+    popup.classList.remove("is-visible");
+    window.setTimeout(() => popup.remove(), 180);
+  }
+
+  window.Warpbox.notify = notify;
+  window.Warpbox.info = function info(message, options) {
+    return notify({ ...(options || {}), variant: "info", message });
+  };
+  window.Warpbox.warning = function warning(message, options) {
+    return notify({ ...(options || {}), variant: "warning", message });
+  };
+  window.Warpbox.error = function error(message, options) {
+    return notify({ ...(options || {}), variant: "error", message });
+  };
+
+  function showGlobalError() {
+    const now = Date.now();
+    if (now - lastGlobalErrorAt < 2500) {
+      return;
+    }
+    lastGlobalErrorAt = now;
+    notify({
+      variant: "error",
+      title: "Page error",
+      message: GENERIC_ERROR_MESSAGE,
+      duration: 9000,
+    });
+  }
+
+  window.addEventListener("error", function (event) {
+    if (event && event.target && event.target !== window) {
+      return;
+    }
+    showGlobalError();
+  });
+
+  window.addEventListener("unhandledrejection", function () {
+    showGlobalError();
+  });
+})();