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/device_summary_test.dart

@@ -0,0 +1,24 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:frontend/model/device_summary.dart';
+
+import '../helpers/fixtures.dart';
+
+void main() {
+  test('DeviceSummary.fromJson maps every count field', () {
+    final summary = DeviceSummary.fromJson(
+      deviceSummaryJson(
+        totalRegistered: 10,
+        seenLastDayRegistered: 4,
+        seenLastDayUnregistered: 2,
+        seenLastWeekRegistered: 8,
+        seenLastWeekUnregistered: 5,
+      ),
+    );
+
+    expect(summary.totalRegistered, 10);
+    expect(summary.seenLastDayRegistered, 4);
+    expect(summary.seenLastDayUnregistered, 2);
+    expect(summary.seenLastWeekRegistered, 8);
+    expect(summary.seenLastWeekUnregistered, 5);
+  });
+}