fix(dashboard): stop ChatPage from clearing all pages' header action buttons

When embedded chat is enabled, ChatPage renders persistently outside
<Routes> but is initially hidden during the plugin-loading window
(~2-4s). Once plugins finish loading, ChatPage mounts for the first time.

Its header-slot effect had early-return branches for !isActive and !narrow
that actively called setEnd(null). Because the user was on /cron, /models,
/sessions, or any non-chat page, this wiped the action buttons that the
current page had already placed in the header.

Affected pages include:
- Cron page — CREATE button disappears
- Models page — 7D/30D/90D filter buttons disappear
- Sessions page — search box disappears

The fix collapses the two early-return branches into one and removes the
setEnd(null) calls. Now ChatPage only sets end when it actually owns the
slot (isActive && narrow), and lets the normal cleanup handle unmounting.

PageHeaderProvider already clears all slots on pathname change via
useLayoutEffect, so ChatPage's active clearing was redundant and harmful.

Fixes #31862

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/src/pages/ChatPage.tsx

@@ -410,14 +410,7 @@ export default function ChatPage({ isActive = true }: { isActive?: boolean }) {
   useEffect(() => {
     // When hidden (non-chat tab) we must not register the header button —
     // another page owns the header's end slot at that point.
-    if (!isActive) {
-      setEnd(null);
-      return;
-    }
-    if (!narrow) {
-      setEnd(null);
-      return;
-    }
+    if (!isActive || !narrow) return;
     setEnd(
       <Button
         ghost