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/menu.ts

@@ -0,0 +1,106 @@
+import { app, Menu, BrowserWindow, shell } from 'electron'
+import type { MenuItemConstructorOptions } from 'electron'
+
+const isMac = process.platform === 'darwin'
+const SKILL_HUB_URL = 'https://skills.cowagent.ai/'
+
+// Send a menu-triggered action to the renderer (e.g. new chat, open settings).
+function emit(win: BrowserWindow | null, action: string) {
+  win?.webContents.send('menu-action', action)
+}
+
+/**
+ * Build a minimal, purpose-built application menu. We intentionally drop most of
+ * Electron's verbose defaults and keep only items that are actually useful for
+ * this app, plus the shortcuts users expect (New Chat, Settings, Reload, etc).
+ */
+export function buildAppMenu(getWindow: () => BrowserWindow | null) {
+  const win = () => getWindow()
+
+  const appMenu: MenuItemConstructorOptions[] = isMac
+    ? [
+        {
+          label: app.name,
+          submenu: [
+            { role: 'about' },
+            { type: 'separator' },
+            { label: 'Settings…', accelerator: 'Cmd+,', click: () => emit(win(), 'open-settings') },
+            { type: 'separator' },
+            { role: 'hide' },
+            { role: 'hideOthers' },
+            { role: 'unhide' },
+            { type: 'separator' },
+            { role: 'quit' },
+          ],
+        },
+      ]
+    : []
+
+  const fileMenu: MenuItemConstructorOptions = {
+    label: 'File',
+    submenu: [
+      { label: 'New Chat', accelerator: 'CmdOrCtrl+N', click: () => emit(win(), 'new-chat') },
+      ...(!isMac
+        ? ([
+            { label: 'Settings', accelerator: 'Ctrl+,', click: () => emit(win(), 'open-settings') },
+            { type: 'separator' },
+            { role: 'quit' },
+          ] as MenuItemConstructorOptions[])
+        : []),
+    ],
+  }
+
+  const editMenu: MenuItemConstructorOptions = {
+    label: 'Edit',
+    submenu: [
+      { role: 'undo' },
+      { role: 'redo' },
+      { type: 'separator' },
+      { role: 'cut' },
+      { role: 'copy' },
+      { role: 'paste' },
+      { role: 'selectAll' },
+    ],
+  }
+
+  const viewMenu: MenuItemConstructorOptions = {
+    label: 'View',
+    submenu: [
+      { role: 'reload' },
+      { role: 'toggleDevTools' },
+      { type: 'separator' },
+      { role: 'resetZoom' },
+      { role: 'zoomIn' },
+      { role: 'zoomOut' },
+      { type: 'separator' },
+      { role: 'togglefullscreen' },
+    ],
+  }
+
+  const windowMenu: MenuItemConstructorOptions = {
+    label: 'Window',
+    submenu: [
+      { role: 'minimize' },
+      ...(isMac ? ([{ role: 'zoom' }] as MenuItemConstructorOptions[]) : []),
+      { type: 'separator' },
+      // Explicit Close so Cmd/Ctrl+W reliably triggers our close-to-tray hide.
+      { label: 'Close Window', accelerator: 'CmdOrCtrl+W', click: () => win()?.close() },
+    ],
+  }
+
+  const helpMenu: MenuItemConstructorOptions = {
+    label: 'Help',
+    submenu: [{ label: 'Skill Hub', click: () => shell.openExternal(SKILL_HUB_URL) }],
+  }
+
+  const template: MenuItemConstructorOptions[] = [
+    ...appMenu,
+    fileMenu,
+    editMenu,
+    viewMenu,
+    windowMenu,
+    helpMenu,
+  ]
+
+  Menu.setApplicationMenu(Menu.buildFromTemplate(template))
+}