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/models/file_pool.dart

@@ -0,0 +1,36 @@
+class FilePool {
+  final String id;
+  final String name;
+  final String? description;
+  final Map<String, dynamic> storageConfig;
+  final Map<String, dynamic> billingConfig;
+  final Map<String, dynamic> policyConfig;
+  final bool isHidden;
+
+  FilePool({
+    required this.id,
+    required this.name,
+    this.description,
+    required this.storageConfig,
+    required this.billingConfig,
+    required this.policyConfig,
+    required this.isHidden,
+  });
+
+  factory FilePool.fromJson(Map<String, dynamic> json) {
+    return FilePool(
+      id: json['id'] as String,
+      name: json['name'] as String,
+      description: json['description'] as String?,
+      storageConfig: json['storage_config'] as Map<String, dynamic>,
+      billingConfig: json['billing_config'] as Map<String, dynamic>,
+      policyConfig: json['policy_config'] as Map<String, dynamic>,
+      isHidden: json['is_hidden'] as bool,
+    );
+  }
+
+  static List<FilePool> listFromResponse(dynamic data) {
+    final parsed = data as List<dynamic>;
+    return parsed.map((e) => FilePool.fromJson(e)).toList();
+  }
+}