fix: watch together bugs, TV focus issues, system locale default

- Fix guest controls locked on join (use reactive context.select)
- Fix Google TV back button when controls hidden (focus recovery + global fallback)
- Fix profile switch screen focus lost on TV (explicit focus request)
- Add 15s host reconnect grace period with reconnecting overlay
- Add participant join/leave notification toasts
- Default to system language on first launch via findDeviceLocale()

lib/services/settings_service.dart

@@ -810,7 +810,7 @@ class SettingsService extends BaseSharedPreferencesService {
 
   AppLocale getAppLocale() {
     final localeString = prefs.getString(_keyAppLocale);
-    if (localeString == null) return AppLocale.en; // Default to English
+    if (localeString == null) return AppLocaleUtils.findDeviceLocale();
 
     return AppLocale.values.firstWhere((locale) => locale.languageCode == localeString, orElse: () => AppLocale.en);
   }