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/css/19-popups.css

@@ -0,0 +1,173 @@
+.warpbox-popups {
+  position: fixed;
+  z-index: 120;
+  inset-block-start: calc(1rem + env(safe-area-inset-top));
+  inset-inline-end: calc(1rem + env(safe-area-inset-right));
+  width: min(26rem, calc(100vw - 2rem));
+  display: grid;
+  gap: 0.75rem;
+  pointer-events: none;
+}
+
+.warpbox-popup {
+  pointer-events: auto;
+  border: 1px solid var(--border);
+  border-radius: calc(var(--radius) - 0.25rem);
+  background: color-mix(in srgb, var(--card) 96%, transparent);
+  color: var(--card-foreground);
+  box-shadow: var(--shadow);
+  opacity: 0;
+  transform: translateY(-0.55rem);
+  transition: opacity 160ms ease, transform 160ms ease;
+  overflow: hidden;
+}
+
+.warpbox-popup.is-visible {
+  opacity: 1;
+  transform: translateY(0);
+}
+
+.warpbox-popup-chrome {
+  display: grid;
+  grid-template-columns: auto minmax(0, 1fr) auto;
+  gap: 0.85rem;
+  align-items: start;
+  padding: 0.95rem;
+}
+
+.warpbox-popup-icon {
+  width: 1.6rem;
+  height: 1.6rem;
+  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-popup-warning .warpbox-popup-icon {
+  background: color-mix(in srgb, var(--primary) 26%, transparent);
+  color: var(--primary-hover);
+}
+
+.warpbox-popup-error .warpbox-popup-icon {
+  background: color-mix(in srgb, var(--danger) 18%, transparent);
+  color: var(--danger);
+}
+
+.warpbox-popup-title {
+  display: block;
+  margin: 0 0 0.18rem;
+  font-size: 0.92rem;
+  line-height: 1.2;
+}
+
+.warpbox-popup-message {
+  margin: 0;
+  color: var(--muted-foreground);
+  font-size: 0.84rem;
+  line-height: 1.45;
+  overflow-wrap: anywhere;
+}
+
+.warpbox-popup-close {
+  min-height: 1.8rem;
+  width: 1.8rem;
+  padding: 0;
+  border-color: var(--border);
+  color: var(--muted-foreground);
+  background: var(--surface-1);
+  font-size: 1rem;
+  line-height: 1;
+}
+
+.warpbox-popup-close:hover {
+  color: var(--foreground);
+  background: var(--surface-1-hover);
+}
+
+.warpbox-popup-actions {
+  display: flex;
+  justify-content: flex-end;
+  gap: 0.55rem;
+  padding: 0 0.95rem 0.95rem;
+}
+
+:root[data-theme="retro"] .warpbox-popups {
+  inset-block-start: 2.65rem;
+  font-family: "PixeloidSans", "PixelOperator", "Microsoft Sans Serif", Tahoma, sans-serif;
+}
+
+:root[data-theme="retro"] .warpbox-popup {
+  border: 1px solid #000000;
+  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, 3px 3px 0 rgba(0, 0, 0, 0.45);
+}
+
+:root[data-theme="retro"] .warpbox-popup::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-popup-error::before {
+  content: "Warpbox - Error";
+}
+
+:root[data-theme="retro"] .warpbox-popup-warning::before {
+  content: "Warpbox - Warning";
+}
+
+:root[data-theme="retro"] .warpbox-popup-info::before {
+  content: "Warpbox - Info";
+}
+
+:root[data-theme="retro"] .warpbox-popup-chrome {
+  padding: 0.8rem;
+}
+
+:root[data-theme="retro"] .warpbox-popup-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-popup-warning .warpbox-popup-icon {
+  color: #9a5b00;
+}
+
+:root[data-theme="retro"] .warpbox-popup-error .warpbox-popup-icon {
+  color: #c00000;
+}
+
+:root[data-theme="retro"] .warpbox-popup-message {
+  color: #000000;
+}
+
+:root[data-theme="retro"] .warpbox-popup-close {
+  width: 1.45rem;
+  height: 1.25rem;
+  min-height: 1.25rem;
+  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: 0.78rem;
+  font-weight: 700;
+}
+
+@media (max-width: 640px) {
+  .warpbox-popups {
+    inset-inline: 1rem;
+    width: auto;
+  }
+}

backend/static/css/20-upload.css

@@ -56,6 +56,10 @@
   display: none !important;
 }
 
+.install-pwa-button[hidden] {
+  display: none !important;
+}
+
 .hero-copy {
   text-align: center;
 }
@@ -395,6 +399,10 @@ button {
   text-align: right;
 }
 
+.upload-file-state-shared {
+  color: var(--primary);
+}
+
 .upload-recovery-overlay {
   position: fixed;
   inset: 0;

backend/static/css/30-download.css

@@ -698,6 +698,12 @@
   text-decoration: none;
 }
 
+.button.is-disabled {
+  opacity: .62;
+  cursor: not-allowed;
+  pointer-events: none;
+}
+
 .upload-processing-alert {
   margin: 1rem 0;
   padding: .85rem 1rem;
@@ -707,6 +713,11 @@
   color: var(--foreground);
 }
 
+.upload-processing-alert-error {
+  border-color: color-mix(in srgb, var(--danger) 55%, transparent);
+  background: color-mix(in srgb, var(--danger) 14%, transparent);
+}
+
 .thumb-link {
   flex: 0 0 4.75rem;
   width: 4.75rem;
@@ -870,6 +881,24 @@
   cursor: wait;
 }
 
+.file-card.is-failed {
+  border-color: color-mix(in srgb, var(--danger) 55%, var(--border));
+  background: color-mix(in srgb, var(--danger) 8%, var(--background));
+}
+
+.file-card.is-failed .file-open {
+  cursor: not-allowed;
+}
+
+.file-error {
+  display: block;
+  max-width: 100%;
+  margin-top: 0.18rem;
+  color: var(--danger);
+  white-space: normal;
+  overflow-wrap: anywhere;
+}
+
 .file-reaction-dock {
   position: static;
   z-index: 2;