feat(desktop): native desktop enhancements

- Add a minimal application menu with common items and shortcuts:
  New Chat (Cmd+N), Settings (Cmd+,), Reload, etc.
- Add system tray with show window, new chat, and quit; click icon to restore
- Add single-instance lock so relaunching focuses the existing window
- Implement close-to-tray: closing the window hides it; only a real quit destroys it
- Explicitly define Window menu's Close Window bound to Cmd+W / Ctrl+W to reliably trigger hide-to-tray
- Listen for menu-action in the renderer to handle menu-triggered new chat / open settings

desktop/src/main/preload.ts

@@ -24,5 +24,10 @@ contextBridge.exposeInMainWorld('electronAPI', {
     ipcRenderer.on('window-maximize-changed', (_event, max) => callback(max))
   },
 
+  // App menu / shortcut actions forwarded from the main process.
+  onMenuAction: (callback: (action: string) => void) => {
+    ipcRenderer.on('menu-action', (_event, action: string) => callback(action))
+  },
+
   platform: process.platform,
 })