A whole bunch of changes

test/theme_toggle_test.dart

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+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:provider/provider.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+import 'package:fuel_tax_tracker/screens/ui_settings_screen.dart';
+import 'package:fuel_tax_tracker/services/app_state.dart';
+
+/// Covers the light/dark/system theme toggle on the Settings > UI screen —
+/// defaults to System, selecting a segment updates [AppState.themeMode]
+/// and persists it to SharedPreferences (so it survives a cold start).
+///
+/// Deliberately doesn't round-trip through a second [AppState.init] to
+/// verify persistence — that pulls in unrelated subsystems (connectivity
+/// listening, cloud-restore) with no test-environment platform mocks,
+/// which hangs rather than failing fast. Reading the pref back directly
+/// verifies the same thing without that baggage.
+void main() {
+  setUp(() {
+    SharedPreferences.setMockInitialValues({});
+  });
+
+  testWidgets('defaults to System and switches to Dark/Light on tap', (tester) async {
+    final appState = AppState()..isLoading = false;
+
+    await tester.pumpWidget(
+      ChangeNotifierProvider.value(
+        value: appState,
+        child: const MaterialApp(home: UiSettingsScreen()),
+      ),
+    );
+
+    expect(appState.themeMode, ThemeMode.system);
+
+    await tester.tap(find.text('Dark'));
+    await tester.pumpAndSettle();
+    expect(appState.themeMode, ThemeMode.dark);
+
+    await tester.tap(find.text('Light'));
+    await tester.pumpAndSettle();
+    expect(appState.themeMode, ThemeMode.light);
+  });
+
+  testWidgets('persists the choice to SharedPreferences', (tester) async {
+    final appState = AppState()..isLoading = false;
+
+    await tester.pumpWidget(
+      ChangeNotifierProvider.value(
+        value: appState,
+        child: const MaterialApp(home: UiSettingsScreen()),
+      ),
+    );
+
+    await tester.tap(find.text('Dark'));
+    await tester.pumpAndSettle();
+
+    final prefs = await SharedPreferences.getInstance();
+    // Mirrors the private `_prefsKeyThemeMode` key in app_state.dart —
+    // a black-box persistence check necessarily has to know the storage
+    // key, and this doubles as a guardrail against silently breaking
+    // already-persisted user settings by renaming it there.
+    expect(prefs.getString('theme_mode'), 'dark');
+  });
+}