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/api_error.dart

@@ -0,0 +1,40 @@
+import 'package:dio/dio.dart';
+import 'package:flutter/foundation.dart';
+
+String dioErrorToUserMessage(Object error) {
+  if (error is TypeError || error is FormatException) {
+    debugPrint('Backend response shape error: $error');
+    return 'Unexpected response shape from backend.';
+  }
+  if (error is! DioException) {
+    debugPrint('Non-Dio error from backend call: $error');
+    return 'Unexpected error.';
+  }
+  switch (error.type) {
+    case DioExceptionType.connectionTimeout:
+    case DioExceptionType.sendTimeout:
+    case DioExceptionType.receiveTimeout:
+      return 'Backend did not respond in time.';
+    case DioExceptionType.connectionError:
+      return 'Cannot reach backend. Check the base URL and your network.';
+    case DioExceptionType.badCertificate:
+      return 'Backend TLS certificate could not be verified.';
+    case DioExceptionType.cancel:
+      return 'Request canceled.';
+    case DioExceptionType.badResponse:
+      final status = error.response?.statusCode;
+      if (status == 401 || status == 403) {
+        return 'Authentication failed. Check your API key in Settings.';
+      }
+      if (status == 404) {
+        return 'Not found.';
+      }
+      if (status != null && status >= 500 && status < 600) {
+        return 'Backend error (status $status). Please try again.';
+      }
+      return 'Unexpected response from backend (status ${status ?? 'unknown'}).';
+    case DioExceptionType.unknown:
+      debugPrint('Unknown Dio error: ${error.message}');
+      return 'Unexpected error contacting backend.';
+  }
+}