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/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;