Rework Dialog focus trap to support nested portals

Context

The current focus trap in Dialog relies on a hand-rolled focusin listener. It breaks as soon as a nested portal (Select, Combobox, DatePicker) renders its content outside the dialog subtree: focus is yanked back to the dialog and the nested widget closes.

Proposed approach

Track an allowlist of portal roots registered through context. Any element inside a registered root is treated as part of the dialog for focus containment purposes.

const PortalRootContext = createContext<Set<HTMLElement>>(new Set());

export function useDialogPortalRoot(node: HTMLElement | null) {
   const roots = useContext(PortalRootContext);
   useEffect(() => {
      if (!node) return;
      roots.add(node);
      return () => void roots.delete(node);
   }, [node, roots]);
}

Acceptance criteria