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/app/sandbox/page.tsx

@@ -224,6 +224,21 @@ export default function SandboxPage() {
     return `${d.getMonth() + 1}/${d.getDate()} ${d.getHours().toString().padStart(2, '0')}:${d.getMinutes().toString().padStart(2, '0')}`;
   }
 
+  async function shareSessionLink() {
+    if (!getToken() || !currentSessionId) { alert('请先登录'); return; }
+    try {
+      const tk = getToken();
+      const res = await fetch(`${API_BASE}/sandbox/sessions/${currentSessionId}/share`, {
+        method: 'POST', headers: { Authorization: `Bearer ${tk}` },
+      });
+      const data = await res.json();
+      if (data.shareUrl) {
+        await navigator.clipboard.writeText(data.shareUrl);
+        alert('链接已复制');
+      }
+    } catch { alert('生成分享链接失败'); }
+  }
+
   function shareToCommunity(content: string, title?: string) {
     if (!getToken()) { alert('请先登录'); return; }
     apiFetch('/community/posts', {
@@ -421,7 +436,11 @@ export default function SandboxPage() {
                 </div>
               )}
               {!sending && messages.length > 2 && (
-                <div className="flex justify-end">
+                <div className="flex justify-end gap-2">
+                  <button onClick={shareSessionLink}
+                    className="text-xs text-brand-600 hover:underline">
+                    {t.sandbox.copyShareLink}
+                  </button>
                   <button onClick={() => {
                     const lastAssistantMsg = [...messages].reverse().find(m => m.role === 'assistant');
                     if (lastAssistantMsg) shareToCommunity(lastAssistantMsg.content);