feat(admin): allow editing boxes and deleting individual files

Introduce new admin capabilities to manage uploaded boxes and files:
- Add routes and handlers for editing boxes and deleting individual files.
- Implement `RemoveFileFromBox` in `UploadService` to delete a file's stored objects and remove it from the box (deleting the box if empty).
- Implement `AdminUpdateBox` in `UploadService` to update expiry, download limits, and clear password protection.
- Remove the unused `AdminFiles` handler.
- Add `.claude` to `.gitignore`.

backend/libs/handlers/app.go

@@ -108,6 +108,9 @@ func (a *App) RegisterRoutes(mux *http.ServeMux) {
 	mux.HandleFunc("POST /admin/users/{userID}/policy", a.AdminUpdateUserPolicy)
 	mux.HandleFunc("POST /admin/users/{userID}/storage", a.AdminUpdateUserStorage)
 	mux.HandleFunc("GET /admin/boxes/{boxID}/view", a.AdminViewBox)
+	mux.HandleFunc("GET /admin/boxes/{boxID}/edit", a.AdminEditBox)
+	mux.HandleFunc("POST /admin/boxes/{boxID}/edit", a.AdminUpdateBox)
+	mux.HandleFunc("POST /admin/boxes/{boxID}/files/{fileID}/delete", a.AdminDeleteBoxFile)
 	mux.HandleFunc("POST /admin/boxes/{boxID}/delete", a.AdminDeleteBox)
 	mux.HandleFunc("GET /d/{boxID}", a.DownloadPage)
 	mux.HandleFunc("GET /d/{boxID}/deleted", a.ManageDeleted)