feat(mui): first ui-kit wrappers (Button, Card, TextField) + barrel

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/admin/ui/TextField.tsx

@@ -0,0 +1,6 @@
+import MuiTextField, { type TextFieldProps } from "@mui/material/TextField";
+
+/** App TextField: small + outlined + full width by default. */
+export default function TextField(props: TextFieldProps) {
+  return <MuiTextField size="small" variant="outlined" fullWidth {...props} />;
+}