Initialize CMS application scaffold

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

ui/app/src/App.tsx

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+import { Navigate, Route, Routes, Link, useLocation, useNavigate } from 'react-router-dom'
+import { useAuth } from './auth'
+
+function Shell({ children, admin = false }: { children: React.ReactNode; admin?: boolean }) {
+  const { user, signOut } = useAuth()
+  return <main className="shell">
+    <header className="header"><Link className="brand" to="/">CMS</Link>{user && !admin && <button className="text-button" onClick={() => void signOut()}>Sign out</button>}</header>
+    {children}
+  </main>
+}
+
+function Login({ admin = false }: { admin?: boolean }) {
+  const { user, loading, error, signIn } = useAuth()
+  const navigate = useNavigate()
+  if (!admin && user) return <Navigate to="/test" replace />
+  if (loading) return <Shell admin={admin}><div className="card"><p>Checking your session…</p></div></Shell>
+  return <Shell admin={admin}><section className="card auth-card">
+    <span className="eyebrow">{admin ? 'Administration' : 'Welcome'}</span>
+    <h1>{admin ? 'Admin sign in' : 'Sign in to CMS'}</h1>
+    <p className="muted">{admin ? 'Use your administrator account to continue.' : 'Sign in securely with your external identity provider.'}</p>
+    {error && <p className="error" role="alert">{error}</p>}
+    {!admin && <button className="google-button" onClick={() => void signIn()}><span aria-hidden="true">G</span> Continue with Google</button>}
+    {admin && <p className="muted small">Administrator authentication is handled by the backend.</p>}
+    {admin && <button className="secondary-button" onClick={() => navigate('/')}>Return to home</button>}
+  </section></Shell>
+}
+
+function ProtectedTest() {
+  const { user, loading } = useAuth()
+  const location = useLocation()
+  if (loading) return <Shell><div className="card"><p>Loading…</p></div></Shell>
+  if (!user) return <Navigate to="/" state={{ from: location }} replace />
+  return <Shell><section className="card"><span className="eyebrow">Authenticated</span><h1>Test page</h1><p>You are signed in and can access protected content.</p><div className="profile">{user.email ?? user.name ? `Signed in as ${user.email ?? user.name}` : 'Your session is active.'}</div></section></Shell>
+}
+
+export function App() {
+  return <Routes>
+    <Route path="/" element={<Login />} />
+    <Route path="/admin/login" element={<Login admin />} />
+    <Route path="/test" element={<ProtectedTest />} />
+    <Route path="*" element={<Navigate to="/" replace />} />
+  </Routes>
+}