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/tray.ts

@@ -0,0 +1,59 @@
+import { app, Tray, Menu, BrowserWindow, nativeImage } from 'electron'
+
+let tray: Tray | null = null
+
+interface TrayDeps {
+  getWindow: () => BrowserWindow | null
+  iconPath?: string
+  // Called when the user picks "Quit" so the app can fully exit.
+  onQuit: () => void
+}
+
+// Build a system tray icon with a minimal menu. The tray lets users restore the
+// window after closing it to the background and start a new chat quickly.
+export function createTray({ getWindow, iconPath, onQuit }: TrayDeps): Tray | null {
+  if (tray) return tray
+  if (!iconPath) return null
+
+  let image = nativeImage.createFromPath(iconPath)
+  if (image.isEmpty()) return null
+  // Tray icons render small; resize to avoid an oversized image on some platforms.
+  image = image.resize({ width: 18, height: 18 })
+  // macOS renders template images correctly in both light/dark menubars.
+  if (process.platform === 'darwin') image.setTemplateImage(false)
+
+  tray = new Tray(image)
+  tray.setToolTip(app.name)
+
+  const showWindow = () => {
+    const win = getWindow()
+    if (!win) return
+    if (win.isMinimized()) win.restore()
+    win.show()
+    win.focus()
+  }
+
+  const contextMenu = Menu.buildFromTemplate([
+    { label: 'Show CowAgent', click: showWindow },
+    {
+      label: 'New Chat',
+      click: () => {
+        showWindow()
+        getWindow()?.webContents.send('menu-action', 'new-chat')
+      },
+    },
+    { type: 'separator' },
+    { label: 'Quit', click: onQuit },
+  ])
+  tray.setContextMenu(contextMenu)
+
+  // Single click restores the window (common Windows/Linux behavior).
+  tray.on('click', showWindow)
+
+  return tray
+}
+
+export function destroyTray() {
+  tray?.destroy()
+  tray = null
+}