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/pods/pool_provider.dart

@@ -0,0 +1,14 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import '../services/pool_service.dart';
+import '../models/file_pool.dart';
+import 'package:island/pods/network.dart';
+
+final poolServiceProvider = Provider<PoolService>((ref) {
+  final dio = ref.watch(apiClientProvider);
+  return PoolService(dio);
+});
+
+final poolsProvider = FutureProvider<List<FilePool>>((ref) async {
+  final service = ref.watch(poolServiceProvider);
+  return service.fetchPools();
+});