docs: add JSDoc to default hub configuration constants

Author: Dark-Brain07

packages/frames.js/src/default.ts

@@ -1,2 +1,11 @@
+/**
+ * Default Farcaster Hub API URL using the Neynar hub service.
+ * Used when no custom hub URL is provided.
+ */
 export const DEFAULT_HUB_API_URL = "https://hub-api.neynar.com";
+
+/**
+ * Default API key for the Neynar hub service.
+ * This is a public API key for frames.js usage.
+ */
 export const DEFAULT_HUB_API_KEY = "NEYNAR_FRAMES_JS";