feat(ground-suite): add theme selector

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,
+};