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

frontend/src/i18n/locales/en.ts

@@ -51,6 +51,8 @@ const en: Translations = {
     notHelpful: 'Not helpful',
     runInCodeSandbox: 'Run in Code Sandbox',
     shareToCommunity: 'Share to Community',
+    copyShareLink: 'Copy Share Link',
+    linkCopied: 'Link copied',
     loginForMore: 'Login for more',
     dailyQuota: '{used} used today, {remaining} remaining',
     aiReplyDisclaimer: 'AI replies are for reference only.',