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/helpers/pump_app.dart

@@ -0,0 +1,60 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:frontend/main.dart';
+import 'package:frontend/theme/catppuccin_mocha_theme.dart';
+import 'package:provider/provider.dart';
+
+/// Pumps a single screen/widget under test inside the minimal scaffolding the
+/// app's widgets expect: an [AppState] provider (read by Settings) and a
+/// [MaterialApp] carrying a real app theme. The real theme is required because
+/// several widgets read `Theme.of(context).extension<AppColorExtension>()!`
+/// non-null and would crash under a bare [ThemeData].
+///
+/// Widgets are pumped directly (never through the router shell) so the AppBar's
+/// Google Fonts lookup is never reached.
+Future<void> pumpScreen(
+  WidgetTester tester,
+  Widget child, {
+  Size size = const Size(900, 1200),
+}) async {
+  tester.view.physicalSize = size;
+  tester.view.devicePixelRatio = 1.0;
+  addTearDown(tester.view.resetPhysicalSize);
+  addTearDown(tester.view.resetDevicePixelRatio);
+
+  await tester.pumpWidget(
+    ChangeNotifierProvider(
+      create: (_) => AppState(),
+      child: MaterialApp(
+        theme: catppuccinMochaDarkTheme,
+        home: Scaffold(body: child),
+      ),
+    ),
+  );
+}
+
+/// Pumps frames (advancing fake time in small steps) until [finder] matches or
+/// [maxFrames] is reached. Needed because Dio resolves each request through an
+/// internal zero-duration timer, so the response only lands once the fake clock
+/// is advanced — `pumpAndSettle` is unusable here as the polling widgets keep a
+/// periodic timer alive forever.
+Future<void> pumpUntilFound(
+  WidgetTester tester,
+  Finder finder, {
+  int maxFrames = 40,
+  Duration step = const Duration(milliseconds: 10),
+}) async {
+  for (var i = 0; i < maxFrames; i++) {
+    if (finder.evaluate().isNotEmpty) return;
+    await tester.pump(step);
+  }
+  if (finder.evaluate().isEmpty) {
+    fail('Timed out waiting for $finder');
+  }
+}
+
+/// Unmounts the widget tree so widgets' `dispose()` cancel their timers and the
+/// test ends without "pending timer" failures. Call at the end of widget tests
+/// that exercise polling widgets.
+Future<void> tearDownTree(WidgetTester tester) =>
+    tester.pumpWidget(const SizedBox());