UI: add back button & improve machine layout

pkgs/webview-ui/app/src/components/BackButton.tsx

@@ -0,0 +1,10 @@
+import { useNavigate } from "@solidjs/router";
+
+export const BackButton = () => {
+  const navigate = useNavigate();
+  return (
+    <button class="btn btn-square btn-ghost" onClick={() => navigate(-1)}>
+      <span class="material-icons ">arrow_back_ios</span>
+    </button>
+  );
+};