fix(12): resolve critical code review findings (CR-01 through CR-04)

- CR-01: fix browse URL from /folders/{id} to /items?parent_ref= (broken feature)
- CR-02: remove raw provider exception text from _TerminalProviderError messages
- CR-03: add user_id predicate to get_or_create_folder_state query (ownership boundary)
- CR-04: add max_length=255 to ConnectionRenameRequest.display_name

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/services/cloud_items.py

@@ -229,6 +229,7 @@ async def get_or_create_folder_state(
     uid_fs = user_id if isinstance(user_id, uuid.UUID) else uuid.UUID(str(user_id))
     result = await session.execute(
         select(CloudFolderState).where(
+            CloudFolderState.user_id == uid_fs,
             CloudFolderState.connection_id == cid_fs,
             CloudFolderState.parent_ref == parent_ref,
         )