Tidy up admin structure

admin/src/lib/components/CenteredPaper.tsx

@@ -0,0 +1,21 @@
+import { Paper } from "@mui/material";
+
+function CenteredPaper({ children }: { children: React.ReactNode }): JSX.Element {
+  return (
+    <div>
+      <Paper
+        elevation={0}
+        style={{
+          display: "flex",
+          justifyContent: "center",
+          alignItems: "center",
+          padding: 36,
+        }}
+      >
+        {children}
+      </Paper>
+    </div>
+  );
+}
+
+export default CenteredPaper;