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/ai/ai-gateway.service.ts

@@ -21,6 +21,7 @@ export interface ModelInfo {
 interface AIProvider {
   name: string;
   chat(messages: ChatMessage[], options?: ChatOptions): Promise<string>;
+  chatStream?(messages: ChatMessage[], options?: ChatOptions): AsyncIterable<string>;
 }
 
 const MODEL_CATALOG: Record<string, ModelInfo> = {
@@ -114,6 +115,54 @@ export class AIGatewayService {
     return this.fallback(messages);
   }
 
+  async *chatStream(model: string, messages: ChatMessage[], options?: ChatOptions): AsyncIterable<string> {
+    const modelMap: Record<string, string> = {
+      'general': 'openai',
+      'openai': 'openai',
+      'gpt-3.5': 'openai',
+      'gpt-4': 'openai',
+      'longcat': 'openai',
+      'meituan/longcat-flash-lite': 'openai',
+      'deepseek-v4-flash': 'deepseek-v4-flash',
+      'sensenova-6.7-flash-lite': 'sensenova-6.7-flash-lite',
+      'sensenova-u1-fast': 'sensenova-u1-fast',
+    };
+
+    const providerKey = modelMap[model.toLowerCase()] || (model.includes('/') ? 'openai' : model);
+    const provider = this.providers.get(providerKey);
+
+    if (!this.usageStats[providerKey]) this.usageStats[providerKey] = { total: 0, success: 0, failed: 0 };
+    this.usageStats[providerKey].total++;
+
+    if (provider?.chatStream) {
+      try {
+        let hasContent = false;
+        for await (const chunk of provider.chatStream(messages, options)) {
+          hasContent = true;
+          yield chunk;
+        }
+        if (!hasContent) throw new Error('AI 返回内容为空');
+        this.usageStats[providerKey].success++;
+      } catch (err: any) {
+        this.logger.error(`${provider.name} 流式调用失败: ${err.message}`);
+        this.usageStats[providerKey].failed++;
+        yield this.fallback(messages);
+      }
+    } else if (provider) {
+      try {
+        const reply = await provider.chat(messages, options);
+        this.usageStats[providerKey].success++;
+        yield reply;
+      } catch (err: any) {
+        this.logger.error(`${provider.name} 调用失败: ${err.message}`);
+        this.usageStats[providerKey].failed++;
+        yield this.fallback(messages);
+      }
+    } else {
+      yield this.fallback(messages);
+    }
+  }
+
   private fallback(messages: ChatMessage[]): string {
     const lastMsg = typeof messages[messages.length - 1]?.content === 'string'
       ? messages[messages.length - 1]?.content as string : '';
@@ -188,6 +237,55 @@ class OpenAICompatibleProvider implements AIProvider {
     }
 
     const data = await res.json() as any;
-    return data.choices[0].message.content;
+    const msg = data.choices[0].message;
+    return msg.content || msg.reasoning_content || '';
+  }
+
+  async *chatStream(messages: ChatMessage[], options?: ChatOptions): AsyncIterable<string> {
+    const res = await fetch(this.apiUrl, {
+      method: 'POST',
+      headers: {
+        'Content-Type': 'application/json',
+        'Authorization': `Bearer ${this.apiKey}`,
+      },
+      body: JSON.stringify({
+        model: this.defaultModel,
+        messages,
+        temperature: options?.temperature ?? 0.7,
+        top_p: options?.top_p ?? 1,
+        max_tokens: options?.max_tokens ?? 2000,
+        stream: true,
+      }),
+    });
+
+    if (!res.ok) {
+      throw new Error(`${this.name} API error: ${res.status} ${await res.text()}`);
+    }
+
+    const reader = res.body!.getReader();
+    const decoder = new TextDecoder();
+    let buffer = '';
+
+    while (true) {
+      const { done, value } = await reader.read();
+      if (done) break;
+
+      buffer += decoder.decode(value, { stream: true });
+      const lines = buffer.split('\n');
+      buffer = lines.pop() || '';
+
+      for (const line of lines) {
+        const trimmed = line.trim();
+        if (!trimmed || !trimmed.startsWith('data:')) continue;
+        const data = trimmed.slice(5).trim();
+        if (data === '[DONE]') return;
+        try {
+          const json = JSON.parse(data);
+          const delta = json.choices?.[0]?.delta || {};
+          const content = delta.content || delta.reasoning_content || '';
+          if (content) yield content;
+        } catch {}
+      }
+    }
   }
 }