Add hub iframe embed shell with tab fragment API.

Replace full-page soft nav with a persistent shell and /api/embed/page loads so tab switches in the hub iframe avoid document.write flicker.

Co-authored-by: Cursor <cursoragent@cursor.com>

static/instance_theme.js

@@ -406,6 +406,7 @@
   /** 中控 iframe：fetch 换页 + 页内遮罩，避免整页卸载与中控侧长时间空白。 */
   function initHubEmbedInFrameNav() {
     if (!isHubLinked()) return;
+    if (document.body && document.body.getAttribute("data-embed-shell") === "1") return;
 
     let navToken = 0;