add default pool selection with validation and fallback

- extend AppSettings with defaultPoolId
- add pool filtering utility to exclude media-only pools
- add resolveDefaultPoolId with fallback to safe pool
- update SettingsScreen with default pool dropdown
- integrate uploadAttachment with default pool resolution

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

lib/utils/pool_utils.dart

@@ -0,0 +1,35 @@
+
+import '../models/file_pool.dart';
+import 'package:hooks_riverpod/hooks_riverpod.dart';
+import '../pods/config.dart';
+
+List<FilePool> filterValidPools(List<FilePool> pools) {
+  return pools.where((p) {
+    final accept = p.policyConfig['accept_types'];
+    if (accept != null) {
+      final acceptsOnlyMedia = accept.every((t) =>
+          t.startsWith('image/') ||
+          t.startsWith('video/') ||
+          t.startsWith('audio/'));
+      if (acceptsOnlyMedia) return false;
+    }
+    return true;
+  }).toList();
+}
+
+String resolveDefaultPoolId(WidgetRef ref, List<FilePool> pools) {
+  final settings = ref.watch(appSettingsNotifierProvider);
+  final validPools = filterValidPools(pools);
+
+  if (settings.defaultPoolId != null &&
+      validPools.any((p) => p.id == settings.defaultPoolId)) {
+    return settings.defaultPoolId!;
+  }
+
+  if (validPools.isNotEmpty) {
+    return validPools.first.id;
+  }
+  // DEFAULT: Solar Network Driver
+  return '500e5ed8-bd44-4359-bc0a-ec85e2adf447';
+}
+