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/application/ports/storage_ports.rs

@@ -168,8 +168,26 @@ pub trait StorageUsagePort: Send + Sync + 'static {
     /// Updates storage usage statistics for a user
     async fn update_user_storage_usage(&self, user_id: &str) -> Result<i64, DomainError>;
 
+    /// Updates storage usage statistics for a user, looked up by username
+    async fn update_user_storage_usage_by_username(
+        &self,
+        username: &str,
+    ) -> Result<i64, DomainError>;
+
     /// Updates storage usage statistics for all users
     async fn update_all_users_storage_usage(&self) -> Result<(), DomainError>;
+
+    /// Checks if a user has enough quota for an additional upload.
+    /// Returns Ok(()) if the upload is allowed, or Err(QuotaExceeded) with a
+    /// descriptive message otherwise.
+    async fn check_storage_quota(
+        &self,
+        user_id: &str,
+        additional_bytes: u64,
+    ) -> Result<(), DomainError>;
+
+    /// Returns (used_bytes, quota_bytes) for a user.
+    async fn get_user_storage_info(&self, user_id: &str) -> Result<(i64, i64), DomainError>;
 }
 
 /// Generic storage service interface for calendar and contact services