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/admin/operations.controller.ts

@@ -107,13 +107,18 @@ export class OperationsController {
   }
 
   @Put('config/:key')
-  async updateConfig(@Param('key') key: string, @Body() body: { value: string }) {
+  async updateConfig(@Param('key') key: string, @Body() body: { value: string; description?: string; category?: string }) {
     const existing = await this.prisma.systemConfig.findUnique({ where: { key } });
     if (existing) {
-      return this.prisma.systemConfig.update({ where: { key }, data: { value: body.value } });
+      return this.prisma.systemConfig.update({ where: { key }, data: { value: body.value, ...(body.description !== undefined ? { description: body.description } : {}) } });
     }
     return this.prisma.systemConfig.create({
-      data: { key, value: body.value, category: 'other' },
+      data: { key, value: body.value, category: body.category || 'other', description: body.description || '' },
     });
   }
+
+  @Delete('config/:key')
+  async deleteConfig(@Param('key') key: string) {
+    return this.prisma.systemConfig.delete({ where: { key } });
+  }
 }