Restore hub iframe soft nav to cut blank tab switch gap.

Use fetch in-frame navigation with overlay and hover prefetch; show delayed hub loading spinner instead of hiding the iframe.

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

manual_trading_hub/static/index.html

@@ -561,6 +561,12 @@
           <button type="button" id="instance-frame-newtab" class="ghost">新标签打开</button>
         </div>
       </div>
+      <div id="instance-frame-loading" class="instance-frame-loading" aria-hidden="true">
+        <div class="instance-frame-loading-inner">
+          <span class="instance-frame-spinner" aria-hidden="true"></span>
+          <span>加载中…</span>
+        </div>
+      </div>
       <iframe id="instance-frame" class="instance-frame" title="交易所实例"></iframe>
     </div>
 
@@ -1040,6 +1046,6 @@
   <script src="/assets/dashboard.js?v=20260612-dash-monitor-count"></script>
   <script src="/assets/ai_review_render.js?v=3"></script>
   <script src="/assets/time_close_ui.js?v=2"></script>
-  <script src="/assets/app.js?v=20260614-calculator"></script>
+  <script src="/assets/app.js?v=20260614-instance-nav"></script>
 </body>
 </html>