feat(ui): theme-mode toggle (system / light / dark) with persistence

Three-way switch in the sidebar footer next to the settings
gear. Cycles system → light → dark → system. Choice persists
via shared_preferences across app launches; restored before the
first frame so there's no theme flicker on startup.

The icon changes per state (auto / sun / moon) and the tooltip
spells it out so the cycling behaviour is discoverable.

- StudioApp is now stateful; exposes StudioApp.of(context) to
  let descendants flip the theme without prop-drilling.
- ThemeModeValue enum lives in data/hub.dart so the persistence
  layer stays free of flutter/material imports.

Bumps fai_studio 0.6.0 -> 0.7.0.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/main.dart

@@ -18,15 +18,53 @@ import 'widgets/widgets.dart';
 
 Future<void> main() async {
   WidgetsFlutterBinding.ensureInitialized();
-  // Restore the persisted endpoint before the first frame so the
-  // sidebar pill doesn't briefly say "127.0.0.1:50051" before
-  // switching to the operator's actual hub.
+  // Restore the persisted endpoint and theme mode before the
+  // first frame so neither flicker on startup.
   await HubService.instance.loadPersistedEndpoint();
-  runApp(const StudioApp());
+  final themeMode = await HubService.instance.loadThemeMode();
+  runApp(StudioApp(initialThemeMode: themeMode));
 }
 
-class StudioApp extends StatelessWidget {
-  const StudioApp({super.key});
+class StudioApp extends StatefulWidget {
+  final ThemeModeValue initialThemeMode;
+
+  const StudioApp({super.key, required this.initialThemeMode});
+
+  /// Lookup helper so descendants can flip the theme without
+  /// passing callbacks down through every level.
+  static StudioAppState? of(BuildContext context) =>
+      context.findAncestorStateOfType<StudioAppState>();
+
+  @override
+  State<StudioApp> createState() => StudioAppState();
+}
+
+class StudioAppState extends State<StudioApp> {
+  late ThemeModeValue _mode;
+
+  ThemeModeValue get mode => _mode;
+
+  Future<void> setMode(ThemeModeValue m) async {
+    setState(() => _mode = m);
+    await HubService.instance.saveThemeMode(m);
+  }
+
+  @override
+  void initState() {
+    super.initState();
+    _mode = widget.initialThemeMode;
+  }
+
+  ThemeMode _flutterMode() {
+    switch (_mode) {
+      case ThemeModeValue.system:
+        return ThemeMode.system;
+      case ThemeModeValue.light:
+        return ThemeMode.light;
+      case ThemeModeValue.dark:
+        return ThemeMode.dark;
+    }
+  }
 
   @override
   Widget build(BuildContext context) {
@@ -35,7 +73,7 @@ class StudioApp extends StatelessWidget {
       debugShowCheckedModeBanner: false,
       theme: FaiTheme.light(),
       darkTheme: FaiTheme.dark(),
-      themeMode: ThemeMode.system,
+      themeMode: _flutterMode(),
       home: const StudioShell(),
     );
   }
@@ -204,20 +242,13 @@ class _Sidebar extends StatelessWidget {
               ],
             ),
           ),
-          // Footer: settings + version.
+          // Footer: theme toggle + settings.
           Padding(
             padding: const EdgeInsets.symmetric(horizontal: FaiSpace.md),
             child: Row(
               children: [
-                Expanded(
-                  child: Text(
-                    'platform target',
-                    style: theme.textTheme.bodySmall?.copyWith(
-                      color: theme.colorScheme.onSurfaceVariant,
-                      fontSize: 10,
-                    ),
-                  ),
-                ),
+                _ThemeToggle(),
+                const Spacer(),
                 IconButton(
                   icon: const Icon(Icons.settings_outlined, size: 16),
                   tooltip: 'Hub endpoint…',
@@ -387,3 +418,34 @@ class _NavPage {
     required this.page,
   });
 }
+
+class _ThemeToggle extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    final app = StudioApp.of(context);
+    final mode = app?.mode ?? ThemeModeValue.system;
+    final icon = switch (mode) {
+      ThemeModeValue.system => Icons.brightness_auto_outlined,
+      ThemeModeValue.light => Icons.light_mode_outlined,
+      ThemeModeValue.dark => Icons.dark_mode_outlined,
+    };
+    final tooltip = switch (mode) {
+      ThemeModeValue.system => 'theme: follow system',
+      ThemeModeValue.light => 'theme: light',
+      ThemeModeValue.dark => 'theme: dark',
+    };
+    return IconButton(
+      icon: Icon(icon, size: 16),
+      tooltip: tooltip,
+      visualDensity: VisualDensity.compact,
+      onPressed: () {
+        final next = switch (mode) {
+          ThemeModeValue.system => ThemeModeValue.light,
+          ThemeModeValue.light => ThemeModeValue.dark,
+          ThemeModeValue.dark => ThemeModeValue.system,
+        };
+        app?.setMode(next);
+      },
+    );
+  }
+}