[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/utils/pool_utils.dart

@@ -1,37 +0,0 @@
-import 'package:island/models/file_pool.dart';
-import 'package:hooks_riverpod/hooks_riverpod.dart';
-import 'package:island/pods/config.dart';
-
-List<SnFilePool> filterValidPools(List<SnFilePool> pools) {
-  return pools.where((p) {
-    final accept = p.policyConfig?['accept_types'];
-
-    if (accept is List) {
-      final acceptsOnlyMedia = accept.every((t) =>
-          t is String &&
-          (t.startsWith('image/') ||
-              t.startsWith('video/') ||
-              t.startsWith('audio/')));
-      if (acceptsOnlyMedia) return false;
-    }
-
-    return true;
-  }).toList();
-}
-
-String resolveDefaultPoolId(WidgetRef ref, List<SnFilePool> pools) {
-  final settings = ref.watch(appSettingsNotifierProvider);
-  final validPools = filterValidPools(pools);
-
-  final configuredId = settings.defaultPoolId;
-  if (configuredId != null &&
-      validPools.any((p) => p.id == configuredId)) {
-    return configuredId;
-  }
-
-  if (validPools.isNotEmpty) {
-    return validPools.first.id;
-  }
-
-  return '500e5ed8-bd44-4359-bc0a-ec85e2adf447'; // Solar Network Driver
-}