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/app.css

@@ -1183,6 +1183,7 @@ body.market-chart-fs-open {
   display: flex;
   flex-direction: column;
   background: var(--bg, #0a0e14);
+  isolation: isolate;
 }
 
 .instance-frame-shell.hidden {
@@ -1190,7 +1191,52 @@ body.market-chart-fs-open {
 }
 
 .instance-frame-shell.is-instance-nav-loading .instance-frame {
-  visibility: hidden;
+  pointer-events: none;
+}
+
+.instance-frame-loading {
+  display: none;
+  position: absolute;
+  left: 0;
+  right: 0;
+  bottom: 0;
+  top: 49px;
+  z-index: 2;
+  align-items: center;
+  justify-content: center;
+  background: color-mix(in srgb, var(--bg, #0a0e14) 72%, transparent);
+  color: var(--muted, #8892b0);
+  font-size: 0.9rem;
+  pointer-events: none;
+}
+
+.instance-frame-shell.is-instance-nav-loading .instance-frame-loading {
+  display: flex;
+}
+
+.instance-frame-loading-inner {
+  display: inline-flex;
+  align-items: center;
+  gap: 10px;
+  padding: 10px 16px;
+  border-radius: 999px;
+  border: 1px solid var(--border-soft);
+  background: color-mix(in srgb, var(--panel-solid) 88%, transparent);
+}
+
+.instance-frame-spinner {
+  width: 16px;
+  height: 16px;
+  border-radius: 50%;
+  border: 2px solid color-mix(in srgb, var(--muted, #8892b0) 35%, transparent);
+  border-top-color: var(--accent, #6eb5ff);
+  animation: instance-frame-spin 0.75s linear infinite;
+}
+
+@keyframes instance-frame-spin {
+  to {
+    transform: rotate(360deg);
+  }
 }
 
 .instance-frame-toolbar {