feat(electron): native notification bridge over contextBridge/IPC

Phase 1 step 3 of VNCprodbuild. electron/preload.ts's contextBridge now
exposes window.vnc.showNotification(title, options), routed via
ipcRenderer.invoke("vnc:show-notification") to a new ipcMain.handle in
electron/main.ts that calls Electron's own Notification API. This is the
desktop shell's native notification path - it sits alongside, not in place
of, the browser/PWA's service-worker push path (public/sw.js's push/
notificationclick handlers + lib/web-push.ts), which is untouched.

lib/electron-bridge.ts gives the renderer a `isElectronShell()` +
`showElectronNotification()` wrapper so app code can detect the shell and
use the native path instead of/alongside SW push - not wired to any real
mail-delivery trigger yet, that's Phase 1 steps 4-6 (JMAP realtime
capability investigation, the background/foreground strategy decision, and
implementing it).

Extended e2e/electron-smoke.spec.ts to prove the IPC plumbing actually
fires end-to-end: calls window.vnc.showNotification from the renderer and
asserts the round-trip resolves (not that a real OS toast appears - not
observable in CI). Verified locally: the call resolves {"shown":true} on
this machine, confirming it genuinely reaches Electron's Notification API
and back, not just that window.vnc exists.

Also fixes a real bug caught by this step's typecheck: the smoke test's
Playwright Page variable was named `window`, shadowing the DOM global
inside every evaluate() callback and silently breaking their types. Renamed
to `appWindow`.

All 4 smoke-test assertions green: npm run build:electron && npm run
test:electron.

e2e/electron-smoke.spec.ts

@@ -19,7 +19,11 @@ const projectRoot = path.resolve(__dirname, '..');
 
 test.describe('Electron desktop shell', () => {
   let electronApp: ElectronApplication;
-  let window: Page;
+  // Named `appWindow`, not `window` - the latter would shadow the DOM
+  // global inside every `appWindow.evaluate(() => window...)` callback
+  // below, silently breaking their typing (evaluate() callbacks run in the
+  // browser context, where `window` must resolve to the DOM global).
+  let appWindow: Page;
   const pageErrors: Error[] = [];
 
   test.beforeAll(async () => {
@@ -38,11 +42,11 @@ test.describe('Electron desktop shell', () => {
       },
     });
 
-    window = await electronApp.firstWindow();
-    window.on('pageerror', (error) => {
+    appWindow = await electronApp.firstWindow();
+    appWindow.on('pageerror', (error) => {
       pageErrors.push(error);
     });
-    await window.waitForLoadState('domcontentloaded');
+    await appWindow.waitForLoadState('domcontentloaded');
   });
 
   test.afterAll(async () => {
@@ -52,11 +56,35 @@ test.describe('Electron desktop shell', () => {
   test('boots the standalone server and renders the login screen', async () => {
     // Same selectors as e2e/login.spec.ts's browser-based check - the
     // shell should render the identical login form, not a different view.
-    await expect(window.locator('input[type="text"]')).toBeVisible({ timeout: 20000 });
-    await expect(window.locator('input[type="password"]')).toBeVisible();
+    await expect(appWindow.locator('input[type="text"]')).toBeVisible({ timeout: 20000 });
+    await expect(appWindow.locator('input[type="password"]')).toBeVisible();
+  });
+
+  test('exposes the contextBridge API to the renderer', async () => {
+    const isElectron = await appWindow.evaluate(() => window.vnc?.isElectron);
+    expect(isElectron).toBe(true);
   });
 
   test('produces zero uncaught page errors', () => {
     expect(pageErrors).toEqual([]);
   });
+
+  test('the native notification bridge round-trips through IPC', async () => {
+    // Not asserting a real OS toast appears - that isn't observable in CI
+    // (headless runners/CI accounts routinely have no notification
+    // permission, and Notification.isSupported() can legitimately be
+    // false). What matters is that window.vnc.showNotification (exposed by
+    // electron/preload.ts's contextBridge) actually reaches the main
+    // process's ipcMain.handle("vnc:show-notification", ...) and resolves -
+    // proving the renderer -> preload -> main -> Electron Notification API
+    // plumbing is wired, not just that `window.vnc` exists.
+    const result = await appWindow.evaluate(async () => {
+      return window.vnc?.showNotification('Electron smoke test', {
+        body: 'IPC round-trip check',
+      });
+    });
+
+    expect(result).toBeDefined();
+    expect(typeof result?.shown).toBe('boolean');
+  });
 });