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.

electron/main.ts

@@ -6,7 +6,7 @@
 // random localhost port, then opens a BrowserWindow pointed at it. This is
 // deliberately the same server, not a reimplementation - lib/jmap/client.ts
 // and every app/api/** route behave identically to the web deployment.
-import { app, BrowserWindow } from "electron";
+import { app, BrowserWindow, ipcMain, Notification } from "electron";
 import { spawn, type ChildProcess } from "node:child_process";
 import { createServer } from "node:net";
 import { get as httpGet } from "node:http";
@@ -131,6 +131,31 @@ async function createMainWindow(): Promise<void> {
   await mainWindow.loadURL(url);
 }
 
+// --- Native notification bridge --------------------------------------------
+// Called from the preload's `window.vnc.showNotification` (electron/preload.ts).
+// Electron's own Notification API is the desktop shell's 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 of the two actually gets wired up to real mail-delivery events is a
+// separate decision (VNCprodbuild Phase 1 steps 4-6); this handler is just
+// the plumbing that lets the renderer trigger a native OS notification at
+// all, so it can be exercised end-to-end from a smoke test now instead of
+// bolted on untested later.
+ipcMain.handle(
+  "vnc:show-notification",
+  (_event, title: string, options?: { body?: string; tag?: string }) => {
+    if (!Notification.isSupported()) {
+      return { shown: false };
+    }
+    const notification = new Notification({
+      title,
+      body: options?.body ?? "",
+    });
+    notification.show();
+    return { shown: true };
+  },
+);
+
 app.whenReady().then(() => {
   void createMainWindow();
 });