feat: P0f 会话分享链接 + 全栈验证

- 后端: POST /sessions/:id/share 生成 HMAC 签名分享令牌
- 后端: GET /sandbox/shared/:token 公开查看分享会话
- 前端: sandbox/page.tsx 新增「复制分享链接」按钮
- 前端: /sandbox/share?token=xxx 分享查看页面
- 全栈: 后端 build + 前端 61 页 + 92 tests 全部通过

backend/src/modules/sandbox/sandbox.module.ts

@@ -1,11 +1,12 @@
 import { Module } from '@nestjs/common';
 import { SandboxController } from './sandbox.controller';
+import { SandboxShareController } from './sandbox-share.controller';
 import { SandboxService } from './sandbox.service';
 import { AIModule } from '../ai/ai.module';
 
 @Module({
   imports: [AIModule],
-  controllers: [SandboxController],
+  controllers: [SandboxController, SandboxShareController],
   providers: [SandboxService],
   exports: [SandboxService],
 })