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/config.dart

@@ -1,11 +1,58 @@
 import 'package:flutter/foundation.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:shared_preferences/shared_preferences.dart';
 
-/// Backend origin. Override at build time:
+const _prefsKey = 'api_base_url';
+
+/// Backend origin when the user has not chosen one. Override at build time:
 /// `flutter run --dart-define=API_BASE_URL=https://fin.example.com`.
 /// On web the default is the page's own origin (Caddy serves app and API together).
-String apiBaseUrl() {
+String defaultApiBaseUrl() {
   const fromEnv = String.fromEnvironment('API_BASE_URL');
   if (fromEnv.isNotEmpty) return fromEnv;
   if (kIsWeb) return Uri.base.origin;
   return 'http://127.0.0.1:8000';
 }
+
+/// The address saved on this device, read in `main()` before `runApp`. Loading it up front,
+/// rather than lazily like the theme, matters: the session restore fires a request on
+/// startup and it must already go to the right server.
+Future<String?> loadStoredApiBaseUrl() async =>
+    (await SharedPreferences.getInstance()).getString(_prefsKey);
+
+/// Overridden in `main()` with [loadStoredApiBaseUrl]; null (tests, nothing saved) means default.
+final initialApiBaseUrlProvider = Provider<String?>((_) => null);
+
+/// Backend origin every client is built against. Watched by `apiProvider` and
+/// `authDioProvider`, so changing it rebuilds them. To switch servers use
+/// `switchApiBaseUrl`: tokens and cached data belong to the old one.
+final apiBaseUrlProvider = NotifierProvider<ApiBaseUrlController, String>(
+  ApiBaseUrlController.new,
+);
+
+class ApiBaseUrlController extends Notifier<String> {
+  @override
+  String build() => ref.watch(initialApiBaseUrlProvider) ?? defaultApiBaseUrl();
+
+  /// [input] as an origin-like URL without a trailing slash, or null if it is not an
+  /// absolute http(s) URL.
+  static String? normalize(String input) {
+    final uri = Uri.tryParse(input.trim());
+    if (uri == null || !uri.hasAuthority || uri.host.isEmpty) return null;
+    if (uri.scheme != 'http' && uri.scheme != 'https') return null;
+    final s = uri.toString();
+    return s.endsWith('/') ? s.substring(0, s.length - 1) : s;
+  }
+
+  /// Persists [url]; null goes back to [defaultApiBaseUrl].
+  Future<void> save(String? url) async {
+    final prefs = await SharedPreferences.getInstance();
+    if (url == null) {
+      await prefs.remove(_prefsKey);
+      state = defaultApiBaseUrl();
+    } else {
+      await prefs.setString(_prefsKey, url);
+      state = url;
+    }
+  }
+}