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-share.controller.ts

@@ -0,0 +1,12 @@
+import { Controller, Get, Param } from '@nestjs/common';
+import { SandboxService } from './sandbox.service';
+
+@Controller('sandbox')
+export class SandboxShareController {
+  constructor(private sandboxService: SandboxService) {}
+
+  @Get('shared/:token')
+  async viewShared(@Param('token') token: string) {
+    return this.sandboxService.getSessionByShareToken(token);
+  }
+}