fix(test): add WebSocket polyfill for Node.js test environment

nostr-tools relay code requires WebSocket which isn't available in
Node.js by default. Adding the ws package polyfill prevents
"ReferenceError: WebSocket is not defined" errors in tests.

src/test/setup.ts

@@ -1,7 +1,13 @@
 /**
  * Vitest setup file
  *
- * Polyfills IndexedDB for Node.js test environment.
- * This allows Dexie to work in tests without a browser.
+ * Polyfills browser APIs for Node.js test environment.
  */
+
+// Polyfill IndexedDB - allows Dexie to work in tests
 import "fake-indexeddb/auto";
+
+// Polyfill WebSocket - required by nostr-tools relay code
+import { WebSocket } from "ws";
+// @ts-expect-error - polyfilling global WebSocket for Node.js
+globalThis.WebSocket = WebSocket;