add pool fetching service and provider

- define FilePool model
- implement PoolService with /drive/pools endpoint
- add Riverpod providers (poolServiceProvider, poolsProvider)

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

lib/services/pool_service.dart

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