feat(app): смена сервера API и очистка данных устройства при выходе

Адрес бэкенда хранится в shared_preferences и читается до runApp; switchApiBaseUrl сбрасывает токены и кэш старого сервера. Выход стирает refresh-токен и sqlite-кэш ответов. Экран настроек переделан под разделы.

app/lib/core/auth/switch_server.dart

@@ -0,0 +1,17 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../config.dart';
+import 'auth_controller.dart';
+
+/// Points the app at [url] (null = the default). A different server means a different
+/// account, so the session ends first — while the clients still talk to the OLD server, so
+/// the refresh token is revoked there and never sent to the new one — and only then is the
+/// address changed. Takes a container rather than a `WidgetRef`: signing out replaces the
+/// calling screen, and a disposed widget's `ref` throws.
+Future<void> switchApiBaseUrl(ProviderContainer container, String? url) async {
+  final next = url ?? defaultApiBaseUrl();
+  if (next != container.read(apiBaseUrlProvider)) {
+    await container.read(authControllerProvider.notifier).logout();
+  }
+  await container.read(apiBaseUrlProvider.notifier).save(url);
+}