feat: AI沙箱流式对话+引导学习+静态服务优化

- 后端: AI沙箱流式对话(SSE), reasoning_content 回退支持
- 前端: 沙箱页流式渲染, 引导学习面板, 默认场景/Starter兜底
- 优化: 前端改用静态文件服务器(node server.js)替代next dev, CSS永不丢失
- 修复: 通用模型默认改为可用模型, predev不再删.next缓存

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

@@ -3,6 +3,14 @@ import { PrismaService } from '../../prisma/prisma.service';
 import { AIGatewayService, ChatOptions } from '../ai/ai-gateway.service';
 import { randomUUID, createHmac } from 'crypto';
 
+interface StreamResult {
+  type: 'text' | 'done' | 'error';
+  content?: string;
+  sessionId?: number;
+  conversationId?: string;
+  message?: string;
+}
+
 @Injectable()
 export class SandboxService {
   constructor(
@@ -78,6 +86,84 @@ export class SandboxService {
     return { reply, conversationId: convId, sessionId: session.id };
   }
 
+  async *chatStream(userId: number, conversationId: string | undefined, model: string, messages: { role: string; content: string }[], options?: ChatOptions, images?: string[]): AsyncGenerator<string> {
+    const user = await this.prisma.user.findUnique({ where: { id: userId } });
+    if (!user || user.status !== 'ACTIVE') {
+      yield JSON.stringify({ type: 'error', message: '用户不可用' } as StreamResult);
+      return;
+    }
+
+    const convId = conversationId || randomUUID();
+
+    const today = new Date();
+    today.setHours(0, 0, 0, 0);
+    const existing = await this.prisma.sandboxSession.findUnique({
+      where: { userId_conversationId: { userId, conversationId: convId } },
+    });
+    if (!existing) {
+      const todayCount = await this.prisma.sandboxSession.count({
+        where: { userId, createdAt: { gte: today } },
+      });
+      if (todayCount >= (user.sandboxDaily || 10)) {
+        yield JSON.stringify({ type: 'error', message: '今日沙箱使用次数已用完' } as StreamResult);
+        return;
+      }
+    }
+
+    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 fullReply = '';
+    try {
+      for await (const chunk of this.aiGateway.chatStream(model, aiMessages, options)) {
+        fullReply += chunk;
+        yield JSON.stringify({ type: 'text', content: chunk } as StreamResult);
+      }
+    } catch (err: any) {
+      yield JSON.stringify({ type: 'error', message: err.message } as StreamResult);
+      return;
+    }
+
+    if (typeof fullReply !== 'string' || fullReply.length === 0) {
+      fullReply = '抱歉，AI 返回了无效的回复，请重试。';
+    }
+
+    const allMessages = messages.concat({ role: 'assistant', content: fullReply });
+    const firstUserMsg = messages.find(m => m.role === 'user');
+    const title = firstUserMsg ? firstUserMsg.content.slice(0, 80) : 'AI 对话';
+
+    const session = await this.prisma.sandboxSession.upsert({
+      where: { userId_conversationId: { userId, conversationId: convId } },
+      create: {
+        userId,
+        conversationId: convId,
+        model,
+        title,
+        messages: JSON.stringify(allMessages),
+        tokens: Math.ceil(fullReply.length / 2),
+      },
+      update: {
+        model,
+        title,
+        messages: JSON.stringify(allMessages),
+        tokens: Math.ceil(fullReply.length / 2),
+      },
+    });
+
+    yield JSON.stringify({ type: 'done', sessionId: session.id, conversationId: convId } as StreamResult);
+  }
+
   async getSessions(userId: number, params: { page?: number; pageSize?: number; search?: string }) {
     const page = Number(params.page ?? 1);
     const pageSize = Number(params.pageSize ?? 50);