Gate push toggle on backend notification method

Add a GET /api/config endpoint exposing the front-end-facing backend
configuration (currently the notification delivery method, grouped under
a nested "notifications" object so the shape can grow). The settings
screen fetches it on init and only shows the per-device push toggle when
the backend method is "push" (and the platform supports push, which keeps
it off the browser).

Also fold in related push-notifications cleanups: fix the Android app
label ("frontend" -> "OOTT") so the notification permission dialog reads
correctly, remove the completed push_notifications.md plan, and update
TODO.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/test/api/config_api_test.dart

@@ -0,0 +1,26 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:frontend/utils/oott_api.dart';
+import 'package:http_mock_adapter/http_mock_adapter.dart';
+
+import '../helpers/backend_test_harness.dart';
+
+void main() {
+  late DioAdapter adapter;
+
+  setUp(() async {
+    adapter = await setUpBackendForTest();
+  });
+
+  test('getConfig GETs /config and decodes the notification method', () async {
+    adapter.onGet(
+      '/config',
+      (server) => server.reply(200, {
+        'notifications': {'method': 'push'},
+      }),
+    );
+
+    final config = await BackendAPI.instance.getConfig();
+
+    expect(config.notificationMethod, 'push');
+  });
+}