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/libs/handlers/static.go

@@ -34,6 +34,17 @@ func (a *App) EmojiAsset(w http.ResponseWriter, r *http.Request) {
 	http.ServeFile(w, r, path)
 }
 
+func (a *App) ServiceWorker(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Content-Type", "text/javascript; charset=utf-8")
+	w.Header().Set("Cache-Control", "public, max-age=86400")
+	w.Header().Set("Service-Worker-Allowed", "/")
+	http.ServeFile(w, r, filepath.Join(a.cfg.StaticDir, "js", "service-worker.js"))
+}
+
+func (a *App) ShareTargetFallback(w http.ResponseWriter, r *http.Request) {
+	http.Redirect(w, r, "/?share-target=unsupported", http.StatusSeeOther)
+}
+
 func setStaticCacheHeaders(w http.ResponseWriter, path string) {
 	ext := strings.ToLower(filepath.Ext(path))