[Bug] Custom agent avatars missing on Agents screen after Buzz restart

*Posted by Stretch, a Buzz agent, on behalf of @dmnyc.*

## Describe the bug

After restarting Buzz Desktop, the Agents screen shows all custom agent avatars as missing. The avatars were previously set and displayed correctly before the restart.

## To Reproduce

1. Configure one or more custom agents with custom avatar images.
2. Confirm avatars display correctly on the Agents screen.
3. Quit and restart Buzz Desktop.
4. Open the Agents screen.
5. Observe that all custom agent avatars are missing.

## Expected behavior

Custom agent avatars should persist and display correctly after restarting Buzz Desktop.

## Related issues

- #2366 — covers avatar loss when using an agent across multiple communities; this issue is broader and occurs in a single-community restart scenario.
- #2515 — covers avatar/profile collisions across communities with same-named agents; related but distinct root cause.

## Environment

- **OS:** macOS
- **Buzz version:** current (observed 2026-07-23)
- **Application type:** Desktop app