fix(c2c): 修复 RangeError 崩溃及订单详情灰色页面问题

根因：多处 [0] 访问空列表/空字符串导致 RangeError 崩溃

- api_client.dart: 后端错误 message 为空数组时 ?[0] 崩溃，改为安全检查
- c2c_order_detail_page.dart: nickname 和 phone 均为 null 时 name[0] 崩溃，
  添加 isNotEmpty 检查和 '未知用户' 回退
- c2c_market_page.dart: makerNickname 为空字符串时 [0] 崩溃，添加 isNotEmpty 检查

这是 C2C 买方"待付款灰色页面"的根本原因：页面在渲染交易双方信息时
因空用户名 RangeError 崩溃，导致整个页面无法显示

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/mining-app/lib/core/network/api_client.dart

@@ -137,7 +137,8 @@ class ApiClient {
         if (statusCode == 401) {
           return UnauthorizedException();
         }
-        final message = e.response?.data?['error']?['message']?[0] ?? '服务器错误';
+        final messages = e.response?.data?['error']?['message'];
+        final message = (messages is List && messages.isNotEmpty) ? messages[0].toString() : '服务器错误';
         return ServerException(message, statusCode: statusCode);
       default:
         return ServerException(e.message ?? '未知错误');

frontend/mining-app/lib/presentation/pages/c2c/c2c_market_page.dart

@@ -314,7 +314,7 @@ class _C2cMarketPageState extends ConsumerState<C2cMarketPage>
                   ),
                   child: Center(
                     child: Text(
-                      (order.makerNickname ?? order.makerPhone ?? 'U')[0].toUpperCase(),
+                      ((order.makerNickname?.isNotEmpty == true ? order.makerNickname : null) ?? (order.makerPhone?.isNotEmpty == true ? order.makerPhone : null) ?? 'U')[0].toUpperCase(),
                       style: TextStyle(
                         fontSize: 18,
                         fontWeight: FontWeight.bold,

frontend/mining-app/lib/presentation/pages/c2c/c2c_order_detail_page.dart

@@ -754,7 +754,9 @@ class _C2cOrderDetailPageState extends ConsumerState<C2cOrderDetailPage> {
           // 发布方
           _buildParticipantRow(
             label: order.isBuy ? '买方 (发布者)' : '卖方 (发布者)',
-            name: order.makerNickname ?? _maskPhone(order.makerPhone ?? ''),
+            name: order.makerNickname?.isNotEmpty == true
+                ? order.makerNickname!
+                : (order.makerPhone?.isNotEmpty == true ? _maskPhone(order.makerPhone!) : '未知用户'),
             isMe: isMaker,
           ),
           const Divider(height: 24),
@@ -762,7 +764,9 @@ class _C2cOrderDetailPageState extends ConsumerState<C2cOrderDetailPage> {
           if (order.takerAccountSequence != null)
             _buildParticipantRow(
               label: order.isBuy ? '卖方 (接单者)' : '买方 (接单者)',
-              name: order.takerNickname ?? _maskPhone(order.takerPhone ?? ''),
+              name: order.takerNickname?.isNotEmpty == true
+                  ? order.takerNickname!
+                  : (order.takerPhone?.isNotEmpty == true ? _maskPhone(order.takerPhone!) : '未知用户'),
               isMe: isTaker,
             )
           else
@@ -791,7 +795,7 @@ class _C2cOrderDetailPageState extends ConsumerState<C2cOrderDetailPage> {
           ),
           child: Center(
             child: Text(
-              name[0].toUpperCase(),
+              name.isNotEmpty ? name[0].toUpperCase() : 'U',
               style: TextStyle(
                 fontSize: 18,
                 fontWeight: FontWeight.bold,