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/unit/friendly_date_formatter_test.dart

@@ -0,0 +1,47 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:frontend/utils/friendly_date_formatter.dart';
+
+void main() {
+  final formatter = FriendlyDateFormatter();
+  final now = DateTime.now();
+
+  test('renders very recent times as "Just now"', () {
+    expect(formatter.format(now.subtract(const Duration(seconds: 30))), 'Just now');
+    expect(formatter.format(now.subtract(const Duration(minutes: 2))), 'Just now');
+  });
+
+  test('renders times under an hour as relative minutes', () {
+    expect(
+      formatter.format(now.subtract(const Duration(minutes: 30))),
+      '30 minutes ago',
+    );
+  });
+
+  test('renders earlier-today / late-yesterday times with a clock time', () {
+    final candidate = now.subtract(const Duration(hours: 2));
+    final sameDay = candidate.year == now.year &&
+        candidate.month == now.month &&
+        candidate.day == now.day;
+
+    expect(
+      formatter.format(candidate),
+      startsWith(sameDay ? 'Today at' : 'Yesterday at'),
+    );
+  });
+
+  test('renders a yesterday-noon time as "Yesterday at"', () {
+    final yesterdayNoon =
+        DateTime(now.year, now.month, now.day - 1, 12, 0);
+
+    expect(formatter.format(yesterdayNoon), startsWith('Yesterday at'));
+  });
+
+  test('renders older dates with the full date', () {
+    final threeDaysAgo = DateTime(now.year, now.month, now.day - 3, 12, 0);
+    final result = formatter.format(threeDaysAgo);
+
+    expect(result, contains(threeDaysAgo.year.toString()));
+    expect(result, isNot(startsWith('Today')));
+    expect(result, isNot(startsWith('Yesterday')));
+  });
+}