fix: soporte visible + perfil con vinculacion de correo

- Setting.fromDocument con valores por defecto (null-safe)
- SettingsProvider crea Setting vacio si el backend falla
- support_view: guard para settings null
- profile_view: seccion 'Acceso con correo' con toggle,
  campos email/password/confirmar, envia OTP, verifica codigo

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/providers/settings_provider.dart

@@ -14,10 +14,13 @@ class SettingsProvider extends ChangeNotifier {
   getSettings() async {
     try {
       final data = await _api.get('/settings');
-      settings = Setting.fromDocument(data as Map<String, dynamic>);
+      if (data != null) {
+        settings = Setting.fromDocument(data as Map<String, dynamic>);
+      }
     } catch (e) {
       print('Error obteniendo los settings: $e');
     } finally {
+      settings ??= Setting.fromDocument({});
       isLoading = false;
       notifyListeners();
     }