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/duration_formatter_test.dart

@@ -0,0 +1,27 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:frontend/utils/duration_formatter.dart';
+
+void main() {
+  test('renders sub-minute values in seconds', () {
+    expect(formatSeconds(0), '0s');
+    expect(formatSeconds(59), '59s');
+  });
+
+  test('rounds fractional seconds to the nearest whole second', () {
+    expect(formatSeconds(5.4), '5s');
+    expect(formatSeconds(5.6), '6s');
+  });
+
+  test('renders minute-and-second values past 60 seconds', () {
+    expect(formatSeconds(60), '1m 0s');
+    expect(formatSeconds(125), '2m 5s');
+  });
+
+  test('rounding up to a full minute carries into the minutes part', () {
+    expect(formatSeconds(59.6), '1m 0s');
+  });
+
+  test('clamps negative values to zero', () {
+    expect(formatSeconds(-10), '0s');
+  });
+}