feat(ground-suite): add theme selector

ground-suite/lib/app/app.dart

@@ -1,19 +1,21 @@
 import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import 'app_shell.dart';
 import 'theme.dart';
+import 'theme_mode_controller.dart';
 
-class GroundSuiteApp extends StatelessWidget {
+class GroundSuiteApp extends ConsumerWidget {
   const GroundSuiteApp({super.key});
 
   @override
-  Widget build(BuildContext context) {
+  Widget build(BuildContext context, WidgetRef ref) {
     return MaterialApp(
       title: 'Ground Suite',
       debugShowCheckedModeBanner: false,
       theme: groundSuiteLightTheme(),
       darkTheme: groundSuiteDarkTheme(),
-      themeMode: ThemeMode.system,
+      themeMode: ref.watch(themeModeProvider),
       home: const AppShell(),
     );
   }

ground-suite/lib/app/app_shell.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
+import 'theme_mode_controller.dart';
 import '../features/diagnostics/presentation/diagnostics_page.dart';
 import '../features/observability/presentation/observability_controller.dart';
 import '../features/overview/presentation/overview_page.dart';
@@ -220,6 +221,7 @@ class _ConnectionBar extends StatelessWidget {
                 _StatusChip(label: state.status.label),
                 Text(peer),
                 Text(frameTime),
+                const _ThemeMenu(),
               ],
             ),
           ),
@@ -229,6 +231,27 @@ class _ConnectionBar extends StatelessWidget {
   }
 }
 
+class _ThemeMenu extends ConsumerWidget {
+  const _ThemeMenu();
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final mode = ref.watch(themeModeProvider);
+
+    return PopupMenuButton<ThemeMode>(
+      tooltip: 'Choose theme',
+      initialValue: mode,
+      onSelected: ref.read(themeModeProvider.notifier).select,
+      icon: Icon(_iconFor(mode)),
+      itemBuilder: (context) => const [
+        PopupMenuItem(value: ThemeMode.system, child: Text('System default')),
+        PopupMenuItem(value: ThemeMode.light, child: Text('Light')),
+        PopupMenuItem(value: ThemeMode.dark, child: Text('Dark')),
+      ],
+    );
+  }
+}
+
 class _CriticalAnnouncement extends StatelessWidget {
   const _CriticalAnnouncement({required this.message});
 
@@ -257,3 +280,9 @@ class _StatusChip extends StatelessWidget {
 }
 
 String _formatTimestamp(DateTime value) => value.toIso8601String();
+
+IconData _iconFor(ThemeMode mode) => switch (mode) {
+  ThemeMode.system => Icons.brightness_auto_outlined,
+  ThemeMode.light => Icons.light_mode_outlined,
+  ThemeMode.dark => Icons.dark_mode_outlined,
+};

ground-suite/lib/app/theme_mode_controller.dart

@@ -0,0 +1,17 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+final themeModeProvider = NotifierProvider<ThemeModeController, ThemeMode>(
+  ThemeModeController.new,
+);
+
+/// Holds the local display preference for the current application session.
+///
+/// It intentionally starts at the system preference. Persisting an operator's
+/// visual preference can be added later without coupling it to vehicle state.
+class ThemeModeController extends Notifier<ThemeMode> {
+  @override
+  ThemeMode build() => ThemeMode.system;
+
+  void select(ThemeMode mode) => state = mode;
+}