fix: address PR review feedback for download cancel/error UI

- Fix transcribe_audio to use whisper-large-v3 mapping (not openai/whisper-large)
- Propagate error field in progress-only fallback path for get_active_tasks
- Use removed return value in cancel endpoint to vary response message
- Add error rollback to handleCancel with toast on failure
- Make isCancelling per-model instead of global
- Fix inverted chevron icons in Problems panel
- Move all clears under lock in clear_all_tasks
- Simplify cancel_download to use dict.pop()

backend/utils/tasks.py

@@ -74,10 +74,7 @@ class TaskManager:
     
     def cancel_download(self, model_name: str) -> bool:
         """Cancel/dismiss a download task (removes it from active list)."""
-        if model_name in self._active_downloads:
-            del self._active_downloads[model_name]
-            return True
-        return False
+        return self._active_downloads.pop(model_name, None) is not None
 
     def is_download_active(self, model_name: str) -> bool:
         """Check if a download is active."""