fix(c2c): 修复付款水单图片无法显示的问题

1. 后端: proofs 图片端点添加 @Public() 装饰器，
   Image.network 无法携带 JWT token
2. 前端: paymentProofUrl 是相对路径，拼接 baseUrl
   构建完整 URL 供 Image.network 加载

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

backend/services/trading-service/src/api/controllers/c2c.controller.ts

@@ -25,6 +25,7 @@ import {
 } from '@nestjs/swagger';
 import * as path from 'path';
 import * as fs from 'fs';
+import { Public } from '../../shared/guards/jwt-auth.guard';
 import { C2cService } from '../../application/services/c2c.service';
 import {
   CreateC2cOrderDto,
@@ -295,6 +296,7 @@ export class C2cController {
     return this.toResponseDto(order);
   }
 
+  @Public()
   @Get('proofs/:filename')
   @ApiOperation({ summary: '获取付款水单图片' })
   @ApiParam({ name: 'filename', description: '文件名' })

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

@@ -3,6 +3,7 @@ import 'package:flutter/services.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
 import 'package:image_picker/image_picker.dart';
+import '../../../core/constants/app_constants.dart';
 import '../../../core/utils/format_utils.dart';
 import '../../../data/models/c2c_order_model.dart';
 import '../../providers/c2c_providers.dart';
@@ -881,6 +882,12 @@ class _C2cOrderDetailPageState extends ConsumerState<C2cOrderDetailPage> {
     );
   }
 
+  /// 将后端返回的相对路径转为完整图片 URL
+  String _buildProofImageUrl(String relativePath) {
+    if (relativePath.startsWith('http')) return relativePath;
+    return '${AppConstants.baseUrl}$relativePath';
+  }
+
   Widget _buildPaymentProofCard(C2cOrderModel order, bool isBuyer) {
     final hasProof = order.paymentProofUrl != null && order.paymentProofUrl!.isNotEmpty;
 
@@ -934,11 +941,11 @@ class _C2cOrderDetailPageState extends ConsumerState<C2cOrderDetailPage> {
           const SizedBox(height: 16),
           if (hasProof)
             GestureDetector(
-              onTap: () => _showProofFullScreen(order.paymentProofUrl!),
+              onTap: () => _showProofFullScreen(_buildProofImageUrl(order.paymentProofUrl!)),
               child: ClipRRect(
                 borderRadius: BorderRadius.circular(8),
                 child: Image.network(
-                  order.paymentProofUrl!,
+                  _buildProofImageUrl(order.paymentProofUrl!),
                   height: 200,
                   width: double.infinity,
                   fit: BoxFit.cover,