Add .jsx filename extension

(This is also required for Vite later)

web/src/components/AvatarBox.jsx

@@ -0,0 +1,25 @@
+import * as React from "react";
+import { Avatar } from "@mui/material";
+import Box from "@mui/material/Box";
+import logo from "../img/ntfy-filled.svg";
+
+const AvatarBox = (props) => {
+  return (
+    <Box
+      sx={{
+        display: "flex",
+        flexGrow: 1,
+        justifyContent: "center",
+        flexDirection: "column",
+        alignContent: "center",
+        alignItems: "center",
+        height: "100vh",
+      }}
+    >
+      <Avatar sx={{ m: 2, width: 64, height: 64, borderRadius: 3 }} src={logo} variant="rounded" />
+      {props.children}
+    </Box>
+  );
+};
+
+export default AvatarBox;