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/test/flutter_test_config.dart

@@ -0,0 +1,12 @@
+import 'dart:async';
+
+import 'package:google_fonts/google_fonts.dart';
+
+/// Global test bootstrap, picked up automatically by `flutter test` for every
+/// test under `test/`. Disables Google Fonts runtime fetching so no test can
+/// accidentally hit the network for a font (tests render screens directly
+/// rather than through the app shell, but this is a cheap safety net).
+Future<void> testExecutable(FutureOr<void> Function() testMain) async {
+  GoogleFonts.config.allowRuntimeFetching = false;
+  await testMain();
+}