fix(examples/hub-next): keep the auth overlay above the dashboard

The AuthOverlay renders as the root div's first child, before header/
main/footer, with position: fixed but no z-index. main uses position:
relative for its absolutely-positioned iframe/panel children, so with
both at z-index: auto the two compete in the same stacking group and
paint order falls back to tree order — main comes later in the DOM and
painted on top, hiding the authorization form behind the dashboard.

Add the shared z-modal-content layer (the same one FixPromptsDialog
uses for its full-screen overlay) so the overlay reliably stacks above
the rest of the page regardless of DOM order.

Author: antfubot

examples/hub-next/src/client/app/page.tsx

@@ -295,7 +295,7 @@ function AuthOverlay({ rpc }: { rpc: DevframeRpcClient }) {
   }
 
   return (
-    <div className="fixed inset-0 grid place-items-center of-auto bg-base p8 color-base">
+    <div className="fixed inset-0 z-modal-content grid place-items-center of-auto bg-base p8 color-base">
       <div className="w-full max-w-100 flex flex-col items-center text-center">
         <div className="grid size-16 place-items-center rounded-2xl bg-active">
           <span className="i-ph-shield-check-duotone text-4xl color-active" />