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/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;