fix(hub): eliminate iframe flash when switching instance nav tabs

Use soft in-frame navigation and loading overlay in hub instance shell; pass embed=1 for iframe SSO opens.

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

manual_trading_hub/static/app.css

@@ -1092,6 +1092,10 @@ body.market-chart-fs-open {
   background: var(--bg);
 }
 
+.instance-frame-shell.is-instance-nav-loading .instance-frame {
+  visibility: hidden;
+}
+
 .exchange-fullscreen {
   position: fixed;
   inset: 0;