Show a reload banner when a new version has deployed (#28)

The service worker already auto-updates in the background (registerType:
'autoUpdate' + an unconditional self.skipWaiting()), but that just
silently swaps the SW -- nothing ever told an already-open tab its
already-loaded JS had fallen behind, so a long-lived tab could run a
stale build indefinitely.

Switched registerType to 'prompt': a new SW now installs and waits
rather than taking over immediately, activating only when the page
explicitly asks (sw.ts's skipWaiting is now conditional on a
SKIP_WAITING message instead of unconditional). UpdateBanner.tsx uses
vite-plugin-pwa's virtual:pwa-register/react hook to surface that as a
small banner with a Reload button, and polls for updates hourly so a
tab that never navigates still notices eventually.

Verified a fresh install shows no banner (correct baseline) and the
code follows the documented registerType: 'prompt' pattern exactly.
Could not get this sandbox's browser to actually detect a swapped
service-worker file via registration.update() during testing --
confirmed via direct inspection that the server serves the new
content correctly and ruled out timing, so this looks like an
update-check limitation specific to this automated browser
environment rather than a bug; the real test is the next live deploy.

Also adds a "frontend-preview" launch.json entry (npm run preview) --
the only way to exercise the real production service worker locally,
same reasoning as vite.config.ts's existing `preview.proxy` section.

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

frontend/src/App.tsx

@@ -2,6 +2,7 @@ import { Navigate, Route, Routes } from 'react-router-dom'
 import { AuthProvider } from './context/AuthContext'
 import { AdminRoute } from './components/AdminRoute'
 import { ProtectedRoute } from './components/ProtectedRoute'
+import { UpdateBanner } from './components/UpdateBanner'
 import { LoginPage } from './pages/LoginPage'
 import { SignupPage } from './pages/SignupPage'
 import { ForgotPasswordPage } from './pages/ForgotPasswordPage'
@@ -12,6 +13,7 @@ import { AdminPage } from './pages/AdminPage'
 function App() {
   return (
     <AuthProvider>
+      <UpdateBanner />
       <Routes>
         <Route path="/login" element={<LoginPage />} />
         <Route path="/signup" element={<SignupPage />} />