feat(desktop): add close-to-tray option

[wade19990814-hue]

Summary

• add a desktop-only closeToTray setting stored in UserConfigDir/reasonix/desktop.json
• initialize a cross-platform system tray with Open / Quit actions only
• intercept window close only when the setting is enabled and a tray is available
• add the setting under Settings → Appearance → Desktop behavior

Closes #3014

Notes

• The setting defaults to off, so existing close-to-exit behavior is unchanged unless users opt in.
• The tray menu intentionally does not expose the setting toggle; it is managed from the desktop Settings panel.
• Windows uses the bundled .ico for the tray icon, while macOS/Linux keep using the PNG app icon.
• Linux checks for a StatusNotifier watcher before enabling close-to-tray, so the app should not hide itself when no tray host is available.
• Windows manual verification covered the Settings entry, the tray icon, and the localized tray menu (打开 / 退出). Screenshots were captured locally by the tester.

ScreenShot

Test plan

• cd desktop && go test -count=1 ./...
• cd desktop/frontend && pnpm exec tsc --noEmit
• cd desktop && wails build