feat: P4 多模态 — 图片上传 + 沙盒 vision 支持

后端:
- ChatMessage 接口支持 text | image_url 多部分内容
- sandbox chat 接受 images[] 参数
- 自动将图片转为 image_url 格式发送到 AI
- 上传模块支持 jpg/png/gif/webp/svg

前端:
- 沙盒输入区 + 图片上传按钮
- 图片缩略图预览 + 删除
- 上传后随消息发送到 API
- uploads/ 目录持久化存储

backend/src/modules/sandbox/sandbox.service.ts

@@ -10,7 +10,7 @@ export class SandboxService {
     private aiGateway: AIGatewayService,
   ) {}
 
-  async chat(userId: number, conversationId: string | undefined, model: string, messages: { role: string; content: string }[], options?: ChatOptions) {
+  async chat(userId: number, conversationId: string | undefined, model: string, messages: { role: string; content: string }[], options?: ChatOptions, images?: string[]) {
     const user = await this.prisma.user.findUnique({ where: { id: userId } });
     if (!user || user.status !== 'ACTIVE') {
       throw new HttpException('用户不可用', HttpStatus.FORBIDDEN);
@@ -33,7 +33,22 @@ export class SandboxService {
       }
     }
 
-    let reply = await this.aiGateway.chat(model, messages as any, options);
+    // Convert images to structured message content
+    let aiMessages = messages as any[];
+    if (images && images.length > 0) {
+      aiMessages = messages.map(m => {
+        if (m.role === 'user' && m === messages[messages.length - 1]) {
+          const parts: any[] = [{ type: 'text', text: m.content }];
+          for (const img of images) {
+            parts.push({ type: 'image_url', image_url: { url: img } });
+          }
+          return { role: m.role, content: parts };
+        }
+        return m;
+      });
+    }
+
+    let reply = await this.aiGateway.chat(model, aiMessages, options);
     if (typeof reply !== 'string') {
       reply = '抱歉，AI 返回了无效的回复，请重试。';
     }