fix(#104): enforce storage quota on uploads & fix usage tracking

Backend:
- Add QuotaExceeded error kind mapped to HTTP 507 Insufficient Storage
- Add check_storage_quota() and get_user_storage_info() to StorageUsagePort
- Enforce quota in upload_file_with_cache, upload_file_with_thumbnails (AuthUser extractor)
- Enforce quota in chunked upload create_upload handler
- Add update_user_storage_usage_by_username() for username-based lookup
- Fix extract_username_from_path() to handle subfolders (take first segment only)
- Fix maybe_update_storage_usage() to use username-based lookup instead of passing
  username to get_user_by_id (which always failed silently)

Frontend:
- Parse and display quota error messages on upload failure (507 / QuotaExceeded)
- Stop remaining uploads when quota is exceeded
- Call refreshUserData() after uploads to update storage usage display
- Bump service worker cache to v9

src/interfaces/errors.rs

@@ -109,6 +109,7 @@ impl From<DomainError> for AppError {
             ErrorKind::NotImplemented => StatusCode::NOT_IMPLEMENTED,
             ErrorKind::UnsupportedOperation => StatusCode::METHOD_NOT_ALLOWED,
             ErrorKind::DatabaseError => StatusCode::INTERNAL_SERVER_ERROR,
+            ErrorKind::QuotaExceeded => StatusCode::INSUFFICIENT_STORAGE,
         };
 
         Self {