feat: implement Portal component and refactor modals to use it for improved positioning

assets/admin/components/ui/Portal.tsx

@@ -0,0 +1,9 @@
+import React, { useEffect, useState } from 'react';
+import { createPortal } from 'react-dom';
+
+export default function Portal({ children }: { children: React.ReactNode }) {
+  const [mounted, setMounted] = useState(false);
+  useEffect(() => setMounted(true), []);
+  if (!mounted) return null;
+  return createPortal(children, document.body);
+}