Fix UniGetUI not quitting properly on macOS

[Gabriel Dufresne (GabrielDuf)]

On macOS, UniGetUI couldn't be quit normally and had to be Force Quit from Activity Monitor. Three distinct issues (#4887):

1. The process never terminated. Quitting only called ApplicationLifetime.Shutdown(), which doesn't stop the background loops (auto-update checker, telemetry, icon database) or the Kestrel IPC host. The process lingered at 2-3% CPU after "closing."
2. Native quit gestures did nothing. Cmd+Q, the application-menu Quit, and Dock → Quit raise the lifetime's ShutdownRequested event, which was unhandled — so they fell through to the hide-to-dock path and the app could never actually be quit.
3. Closing to the Dock was a dead end. Once the window was hidden, clicking the Dock icon never brought it back.

Changes

• Single hard-exit quit path. All real quits now funnel through QuitApplication(), which releases window resources (saves geometry, disposes the tray icon) and hard-exits via Environment.Exit(0) after stopping the IPC server — mirroring the WinUI app's existing StopIpcAndExitAsync. A re-entrancy guard (IsQuitting) prevents double-teardown.
• Handle ShutdownRequested in App so macOS quit gestures (Cmd+Q, app menu, Dock → Quit) and OS session-end route into the clean quit path.
• Handle IActivatableLifetime Reopen so clicking the Dock icon restores a hidden window.
• Platform-correct close button. On macOS the close button always hides the window to the Dock (the platform standard) regardless of the tray setting; the app is quit via Cmd+Q / app menu / Dock. On Windows/Linux, close-to-tray is unchanged when the tray is enabled, and closing with the tray disabled now fully terminates the process instead of leaving background threads alive.
• The in-app Quit (PageType.Quit) and app-restart helper already route through the same path.

Cross-platform safety

• Environment.Exit(0) loses no data: logs, settings, and operation history all write synchronously, and window geometry is saved continuously. This matches what the WinUI app already does.
• The new ShutdownRequested handler doesn't affect the normal Windows/Linux flow — Avalonia's OnLastWindowClose calls Shutdown() internally without raising ShutdownRequested.
• The Dock-reopen handler is a no-op off macOS (TryGetFeature() is guarded; Reopen is macOS-only).
• The tray icon is explicitly disposed before exit to avoid a ghost notification-area icon on Windows.