Split oott_api.dart into per-domain modules

Break the 372-line frontend API client into focused files under
lib/utils/api/: error mapping (api_error.dart), Dio setup behind a
buildDio factory (dio_config.dart), and device/scanner/notification
endpoints as extensions in part files. Collapse the five near-identical
scanner-status methods via a generic _getModel helper and de-duplicate
the pagination logic via a shared _paginate helper.

The BackendAPI singleton and dioErrorToUserMessage remain importable
from oott_api.dart unchanged, so no call sites are affected.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/lib/utils/api/oott_api_notifications.dart

@@ -0,0 +1,39 @@
+part of '../oott_api.dart';
+
+/// Notification endpoints: listing and read/new state transitions.
+extension NotificationApi on BackendAPI {
+  Future<void> markNotificationAsRead(int id) async {
+    await _dio.get('/notifications/$id');
+  }
+
+  Future<void> markNotificationAsNew(int id) async {
+    await _dio.post('/notifications/$id/mark_as_new');
+  }
+
+  Future<void> markAllNotificationsAsRead() async {
+    await _dio.post('/notifications/mark_all_as_old');
+  }
+
+  Future<({List<Notification> items, bool hasNextPage})> listNotifications(
+    bool? isNew, {
+    int page = 0,
+    int perPage = 5,
+    CancelToken? cancelToken,
+  }) async {
+    final response = await _dio.get(
+      '/notifications',
+      queryParameters: {
+        'is_new': isNew ?? '',
+        'page_offset': page * perPage,
+        'page_limit': perPage + 1,
+      },
+      cancelToken: cancelToken,
+    );
+
+    return _paginate(
+      response.data as List<dynamic>,
+      (item) => Notification.fromJson(item),
+      perPage,
+    );
+  }
+}