fix: infinite loop on user-initiated exit

[velocitysystems]

Problem

User-initiated app exit (e.g. closing the window, Cmd+Q) could hang indefinitely. The exit handler called api.prevent_exit(), ran cleanup, then called app_handle.exit() — which re-triggered ExitRequested, creating an infinite loop.

Solution

Break the loop by ensuring cleanup only runs once, then letting the programmatic exit through:

1. An atomic state guard tracks whether cleanup has started. The first ExitRequested claims ownership; subsequent ones are skipped.
2. Cleanup runs asynchronously on the Tokio runtime instead of blocking the event loop with thread::spawn().join().
3. After cleanup finishes (or times out after 5 seconds), app_handle.exit(0) fires a programmatic exit. The handler recognises programmatic exits by their exit code and lets them pass through without re-running cleanup.
4. If the user triggers exit again while cleanup is still running (e.g. repeated Cmd+Q on macOS), prevent_exit() is called to keep the app alive until cleanup completes, but cleanup itself is not restarted.