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/Button.tsx

@@ -0,0 +1,6 @@
+import MuiButton, { type ButtonProps } from "@mui/material/Button";
+
+/** App Button: defaults to the brand primary contained style. */
+export default function Button(props: ButtonProps) {
+  return <MuiButton variant="contained" color="primary" {...props} />;
+}