fix: seed production defaults and harden onboarding

frontend/lib/shared/services/guest_merge_service.dart

@@ -39,15 +39,17 @@ class GuestMergeService {
         : (existingLedger['id'] as num).toInt();
     final categoryMap = <int, int>{};
     final available = <Map<String, dynamic>>[];
-    for (final type in ['expense', 'income']) {
+    for (final type in ['expense', 'income']) {
       final response = await _dio.get(
         '/api/categories',
         queryParameters: {'type': type},
       );
       available.addAll(
         (response.data as List).map((item) => item as Map<String, dynamic>),
-      );
-    }
+      );
+    }
+    await _ensureFallbackCategory(available, 'expense');
+    await _ensureFallbackCategory(available, 'income');
 
     for (final value in snapshot['categories'] as List<dynamic>? ?? const []) {
       final category = value as Map<String, dynamic>;
@@ -90,9 +92,10 @@ class GuestMergeService {
             item['name'] == transaction['categoryName'],
       );
       if (exact.isNotEmpty) return (exact.first['id'] as num).toInt();
-      final fallback = available.firstWhere(
-        (item) => item['type'] == categoryType && item['name'] == '其他',
-      );
+      final fallback = available.firstWhere(
+        (item) => item['type'] == categoryType && item['name'] == '其他',
+        orElse: () => throw StateError('$categoryType 分类缺少“其他”，无法导入本机账单'),
+      );
       return (fallback['id'] as num).toInt();
     }
 
@@ -145,13 +148,47 @@ class GuestMergeService {
 
     await CurrentLedgerStore.instance.ensureLoaded(force: true);
     await CurrentLedgerStore.instance.select(ledgerId);
-    return GuestMergeResult(
-      ledgerId: ledgerId,
-      transactionCount: transactionCount,
-    );
-  }
-
-  static int? _findMappedDefault(
+    return GuestMergeResult(
+      ledgerId: ledgerId,
+      transactionCount: transactionCount,
+    );
+  }
+
+  static Future<void> _ensureFallbackCategory(
+    List<Map<String, dynamic>> available,
+    String type,
+  ) async {
+    bool hasFallback() => available.any(
+      (item) => item['type'] == type && item['name'] == '其他',
+    );
+    if (hasFallback()) return;
+
+    try {
+      final response = await _dio.post(
+        '/api/categories',
+        data: {
+          'name': '其他',
+          'iconKey': 'tag',
+          'colorKey': type == 'income' ? 'lime' : 'graphite',
+          'type': type,
+        },
+      );
+      available.add(response.data as Map<String, dynamic>);
+    } catch (_) {
+      // Another request may have created it between the list and create calls.
+      final response = await _dio.get(
+        '/api/categories',
+        queryParameters: {'type': type},
+      );
+      available.removeWhere((item) => item['type'] == type);
+      available.addAll(
+        (response.data as List).map((item) => item as Map<String, dynamic>),
+      );
+      if (!hasFallback()) rethrow;
+    }
+  }
+
+  static int? _findMappedDefault(
     List<Map<String, dynamic>> available,
     Map<String, dynamic> snapshot,
     int oldCategoryId,