feat: add polling fallback for stale asset downloads (#7926)

## Summary

Adds a polling fallback mechanism to recover from dropped WebSocket
messages during model downloads.

## Problem

When downloading models via the asset download service, status updates
are received over WebSocket. Sometimes these messages are dropped
(network issues, reconnection, etc.), causing downloads to appear
"stuck" even when they've completed on the backend.

## Solution

Periodically poll for stale downloads using the existing REST API:

- Track `lastUpdate` timestamp on each download
- Downloads without updates for 10s are considered "stale"
- Poll stale downloads every 10s via `GET /tasks/{task_id}` to check if
the asset exists
- If the asset exists with size > 0, mark the download as completed

## Implementation

- Added `lastUpdate` field to `AssetDownload` interface
- Use VueUse's `useIntervalFn` with a `watch` to auto start/stop polling
based on active downloads
- Reuse existing `handleAssetDownload` for completion (synthetic event)
- Added 9 unit tests covering the polling behavior

## Testing

- All existing tests pass
- New tests cover:
  - Basic download tracking
  - Completion/failure handling  
  - Duplicate message prevention
  - Stale download polling
  - Polling error handling

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7926-feat-add-polling-fallback-for-stale-asset-downloads-2e36d73d3650810ea966f5480f08b60c)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Amp <amp@ampcode.com>
Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

AGENTS.md

@@ -161,6 +161,8 @@ The project uses **Nx** for build orchestration and task management
 
 ## Testing Guidelines
 
+See @docs/testing/*.md for detailed patterns.
+
 - Frameworks:
   - Vitest (unit/component, happy-dom)
   - Playwright (E2E)