UI: chore create prettier config. Avoid ci/local formatting discprepancies

pkgs/webview-ui/app/prettier.config.js

@@ -0,0 +1,9 @@
+/**
+ * @see https://prettier.io/docs/en/configuration.html
+ * @type {import("prettier").Config}
+ */
+const config = {
+  trailingComma: "all",
+};
+
+export default config;