refactor(frontend): split endpoint handling

frontend/src/utils/api/client.ts

@@ -0,0 +1,12 @@
+const API_URL = import.meta.env.VITE_API_URL;
+
+class AuthError extends Error {
+    constructor() { super("Not authenticated"); this.name = "AuthError"; }
+}
+
+const isThumbnailAvailable = async (thumbnailUrl: string): Promise<boolean> => {
+    const response = await fetch(thumbnailUrl, { credentials: 'include' });
+    return response.ok;
+};
+
+export { API_URL, AuthError, isThumbnailAvailable };