fix: guard hotkey close(nil) panic, non-blocking tray sends, D-Bus leak cleanup

[bosiakov]

• Guard nativeBackend.Unregister() against close(nil) panic
• Make tray menu sends non-blocking to prevent deadlock
• Clean up hotkey backends on failed registration
• GNOME D-Bus connection cleanup on Register() failure
• Fix portal goroutine leak with done channel in listenActivated
• Add portal session cleanup on BindShortcuts failure
• Make MarkClosed() idempotent for Wayland hide/close flow
• Protect dispatch_darwin startFunc with sync.Once
• Clamp config HotkeyPreset to valid range on load
• Quote Exec= path in Linux autostart desktop entry