[REF] unify pool handling with extension methods

- Move pool filtering and parsing logic into SnFilePool extension
- Replace PoolService and pool_utils with unified extension
- Update settings screen to use pools.filterValid() + resolveDefaultPoolId
- Cleanup references in compose_shared.dart
- Remove obsolete files: pool_service.dart, pool_utils.dart

Signed-off-by: Texas0295 <kimura@texas0295.top>

lib/services/pool_service.dart

@@ -1,19 +0,0 @@
-import 'package:dio/dio.dart';
-
-import 'package:island/models/file_pool.dart';
-
-class PoolService {
-  final Dio _dio;
-
-  PoolService(this._dio);
-
-  Future<List<SnFilePool>> fetchPools() async {
-    final response = await _dio.get('/drive/pools');
-
-    if (response.statusCode == 200) {
-      return SnFilePoolList.listFromResponse(response.data);
-    } else {
-      throw Exception('Failed to fetch pools: ${response.statusCode}');
-    }
-  }
-}