feat(admin): bearer auth with admin-only gate + login screen

apps/admin/src/App.tsx

@@ -1,3 +1,20 @@
-export default function App() {
+import { AuthProvider, useAuth } from './auth/AuthContext';
+import Login from './screens/Login';
+
+function AuthedShell() {
+  // Placeholder shell — replaced by the sidebar + routing in Task 4.
   return <div>SoleLog Admin</div>;
 }
+
+function Gate() {
+  const { isAuthed } = useAuth();
+  return isAuthed ? <AuthedShell /> : <Login />;
+}
+
+export default function App() {
+  return (
+    <AuthProvider>
+      <Gate />
+    </AuthProvider>
+  );
+}