feat: add Check for Updates command to native menu and Cmd+K palette

- Add APP_CHECK_FOR_UPDATES constant and menu item in menu.rs (between About and Settings)
- Wire onCheckForUpdates handler through useMenuEvents and useAppCommands
- Add test for app-check-for-updates dispatch

src/hooks/useAppCommands.ts

@@ -122,6 +122,7 @@ export function useAppCommands(config: AppCommandsConfig): CommandAction[] {
     onSearch: config.onSearch,
     onGoBack: config.onGoBack,
     onGoForward: config.onGoForward,
+    onCheckForUpdates: config.onCheckForUpdates,
     activeTabPathRef: config.activeTabPathRef,
     handleCloseTabRef: config.handleCloseTabRef,
     activeTabPath: config.activeTabPath,