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/data/hub.dart

@@ -52,6 +52,22 @@ class HubService {
     await prefs.setBool(_kSecureKey, endpoint.secure);
   }
 
+  static const _kThemeKey = 'theme.mode';
+
+  /// Read the persisted theme mode (system / light / dark) for
+  /// initial app startup. Defaults to system.
+  Future<ThemeModeValue> loadThemeMode() async {
+    final prefs = await SharedPreferences.getInstance();
+    final raw = prefs.getString(_kThemeKey);
+    return ThemeModeValue.fromWire(raw) ?? ThemeModeValue.system;
+  }
+
+  /// Persist the theme mode the user chose.
+  Future<void> saveThemeMode(ThemeModeValue mode) async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString(_kThemeKey, mode.wire);
+  }
+
   Future<bool> healthy() => _client.healthy();
 
   Future<List<ModuleSummary>> listModules() async {
@@ -216,6 +232,24 @@ class ServiceEntry {
   });
 }
 
+/// Three-way switch persisted across launches.
+enum ThemeModeValue {
+  system('system'),
+  light('light'),
+  dark('dark');
+
+  final String wire;
+  const ThemeModeValue(this.wire);
+
+  static ThemeModeValue? fromWire(String? s) {
+    if (s == null) return null;
+    for (final v in values) {
+      if (v.wire == s) return v;
+    }
+    return null;
+  }
+}
+
 /// UI-side type, decoupled from the proto wire type so pages
 /// don't import protobuf packages.
 class ModuleSummary {