feat(avalonia): implement preview hotkey setting and update migration checklist

- Add PreviewHotkey property to HotkeySettingsViewModel
- Add preview hotkey UI to HotkeySettingsPage
- Implement dynamic hotkey handling in MainWindow code-behind
- Remove hardcoded F1 keybinding from XAML
- Update AVALONIA_MIGRATION_CHECKLIST.md with accurate progress:
  - Overall progress: 60-65% (was 35-40%)
  - Plugin Store: 95% done (was 0%)
  - Plugin Settings: 90% done (was 33%)
  - Proxy Settings: 100% done (was 59%)

💘 Generated with Crush

Assisted-by: Kimi for Coding via Crush <[email protected]>

Author: taooceros