Add automated test suite for the Flutter frontend

Introduce a headless `flutter test` suite (83 tests) covering models,
utilities, every API endpoint, and the five screens, with shared helpers
and fixtures so new tests stay terse.

API endpoint methods are statically-dispatched extensions on the
BackendAPI singleton, so they cannot be mocked via `implements`. Mock at
the Dio HTTP-adapter layer (http_mock_adapter) instead, enabled by two
small @visibleForTesting seams: BackendAPI.dioForTesting swaps the
singleton's Dio, and BackendReachability.forceOnlineForTesting() forces a
deterministic online state so polling widgets load.

Add frontend/run_tests.sh and document it in CLAUDE.md.

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

frontend/lib/utils/oott_api.dart

@@ -2,6 +2,7 @@ library;
 
 import 'package:dio/dio.dart';
 import 'package:encrypter/encrypter/xor.dart';
+import 'package:flutter/foundation.dart';
 
 import 'api/api_error.dart';
 import 'api/dio_config.dart';
@@ -45,6 +46,18 @@ class BackendAPI {
     BackendReachability.instance.setProber(() => _dio.get('/test'));
   }
 
+  /// Test-only access to the underlying [Dio] so tests can swap in a client
+  /// with a mock adapter installed. Mirrors [reconfigureFromPrefs]'s prober
+  /// wiring so reachability stays consistent after a swap.
+  @visibleForTesting
+  Dio get dioForTesting => _dio;
+
+  @visibleForTesting
+  set dioForTesting(Dio dio) {
+    _dio = dio;
+    BackendReachability.instance.setProber(() => _dio.get('/test'));
+  }
+
   // Returns null if the test was successful, and a String with a message about the issue if not
   static Future<String?> test(String baseUrl, String apiKey) async {
     final dio = buildDio(