fix(ui): mount only the active session chat iframe (#2816)

* fix(ui): mount only the active session chat iframe

* repro(ui): mount-all-persisted session-chat iframes (#2815)

Adds a regression-style reproduction for issue #2815: ContextPanel
renders one full-application iframe for every chat tab (inactive tabs
are only hidden via the Tailwind 'hidden' class, never unmounted), so a
reload restores all persisted session-chat tabs from the ui-store and
mounts N embedded OpenChamber apps in one browser tab.

The test reads the real ContextPanel.tsx render block, drives the real
useUIStore with the issue's persisted scenario (11 tabs, 8 read-only
session-chat tabs), and models the render block with the real
buildEmbeddedSessionChatURL helper, showing 8 live src iframes (7
hidden but loaded).

* test(ui): adapt issue 2815 reproduction for active chat

* fix(ui): unmount session chat when panel closes

---------

Co-authored-by: ChangeHow <23733347+ChangeHow@users.noreply.github.com>

packages/ui/src/components/layout/contextPanelEmbeddedChat.ts

@@ -158,6 +158,17 @@ export const getOrCreateEmbeddedSessionChatURL = (
   return src;
 };
 
+export const getActiveEmbeddedSessionChatTab = <T extends { id: string }>(
+  tabs: T[],
+  activeTabID: string | null,
+): T | null => {
+  if (!activeTabID) {
+    return null;
+  }
+
+  return tabs.find((tab) => tab.id === activeTabID) ?? null;
+};
+
 /**
  * True when the current document is the embedded session-chat iframe
  * (`?ocPanel=session-chat`). Used to distinguish the embedded iframe from