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/app.go

@@ -54,6 +54,8 @@ func (a *App) renderPage(w http.ResponseWriter, r *http.Request, status int, pag
 func (a *App) RegisterRoutes(mux *http.ServeMux) {
 	mux.HandleFunc("GET /", a.Home)
 	mux.HandleFunc("GET /api", a.APIDocs)
+	mux.HandleFunc("GET /service-worker.js", a.ServiceWorker)
+	mux.HandleFunc("POST /share-target", a.ShareTargetFallback)
 	mux.HandleFunc("GET /register", a.Register)
 	mux.HandleFunc("POST /register", a.RegisterPost)
 	mux.HandleFunc("GET /login", a.Login)