[feature] Minimize to background instead of quitting

[Ampedeath]

Like Steam: closing the main window should hide it, not exit the app. The app keeps running so reminders keep firing; Ctrl+Shift+N (already wired to show/focus the main window) brings it back.

• Intercept the window close event (on_window_event / CloseRequested) and call .hide() instead of letting it exit
• Add a system tray icon (tauri::tray) with at least Show and Quit menu items — Quit is needed since the window's own close button no longer exits
• Verify Ctrl+Shift+N still restores the window from this hidden state
• Pairs well with the existing autostart toggle (start hidden in tray on login, instead of popping the window open)