feat(desktop): in-app prompt to restart for an update

The main process already showed a system notification when an update finished
downloading, and nothing in the renderer listened — so the only cue vanished on
its own and was missed by anyone not watching the desktop at that moment.

A prompt now stays in the window until it is acted on or dismissed, with the
restart wired to the install-update channel the preload already exposed.

Dismissing is 'not now', not 'never': a later version reopens the prompt, since
the version it refers to has changed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/web/src/components/layout/AppLayout.tsx

@@ -1,4 +1,5 @@
 import React, { useEffect } from 'react';
+import { UpdateBanner } from '../ui/UpdateBanner';
 import { useExpressionStore } from '../../stores/expressionStore';
 import { useParams, useNavigate } from 'react-router-dom';
 import { SpaceSidebar } from './SpaceSidebar';
@@ -428,6 +429,7 @@ export function AppLayout() {
             (e.g. immediately after Join, or after popping voice-full back to
             the root). */}
         <SoundController />
+        <UpdateBanner />
         <GlobalAudioRenderer />
         <NotificationController />
         <UpdateToast />