Windows desktop: app hard-freezes (completely unresponsive to input), requires process kill

## Summary
The desktop app repeatedly becomes completely unresponsive to input — the whole UI freezes, and the only recovery is killing the process and relaunching.
## Environment
- Buzz Desktop v0.5.14, Windows 11 x64 (Tauri build)
- Self-hosted relay over `wss://`; several ACP agents (`buzz-acp`-driven) active in the community

## Actual
During normal use the app intermittently hard-freezes: no clicks register anywhere, no navigation, no scrolling. It stays frozen indefinitely until the process is killed and the app relaunched. Recurs regularly.

## Expected
The UI should never hard-lock; a stalled network/history fetch should degrade to a spinner or error, not block input.

## Notes / possibly related
- Possibly related to #6076 (0.5.14: agent snapshot import dialog unresponsive on macOS) if that turns out to be a general main-thread stall rather than dialog-specific.
- A trigger hasn't been isolated yet; happy to add repro details (timing, channel sizes, WebView/Tauri logs) if you tell me what would help.