feat: Electron desktop app — hardening, IPC bridge, and dev launch fixes

- Dev/prod URL auto-detection (Vite 5173 in dev, server 3000 in prod)
- Typed IPC bridge via preload (notifications, badge, window controls, updates, deep links)
- Native OS notifications via NotificationController with window focus suppression
- Auto-update via electron-updater with UpdateToast UI
- Deep linking (backspace:// protocol) for macOS and Windows/Linux
- Window state persistence (position, size, maximize across restarts)
- Tray icon with graceful fallback when icon asset missing
- Suppress PWA service worker polling/reloads inside Electron
- Platform detection layer (isElectron, getElectronAPI)
- Root workspace scripts (dev:desktop, build:desktop)
- Document BACKSPACE_URL and BACKSPACE_UPDATE_URL env vars

packages/web/src/platform/electron.d.ts

@@ -0,0 +1,32 @@
+/** Type augmentation for the Electron IPC bridge exposed by preload.ts */
+
+interface BackspaceElectronAPI {
+  // Platform info
+  platform: NodeJS.Platform;
+
+  // Window controls
+  minimize: () => void;
+  maximize: () => void;
+  close: () => void;
+
+  // Notifications & badge
+  showNotification: (title: string, body: string) => void;
+  setBadgeCount: (count: number) => void;
+
+  // Auto-update (Task 2.1)
+  onUpdateAvailable: (callback: (info: { version: string }) => void) => void;
+  onUpdateDownloaded: (callback: (info: { version: string }) => void) => void;
+  onUpdateError: (callback: (error: string) => void) => void;
+  installUpdate: () => void;
+  checkForUpdates: () => void;
+
+  // Window focus (Task 2.2)
+  onWindowFocusChange: (callback: (focused: boolean) => void) => void;
+
+  // Deep linking (Task 2.3)
+  onDeepLink: (callback: (url: string) => void) => void;
+}
+
+interface Window {
+  backspace?: BackspaceElectronAPI;
+}