fix: remove hardcoded baseURL from auth client

NEXT_PUBLIC_ vars are baked in at build time, so NEXT_PUBLIC_APP_URL=localhost
was being embedded in the bundle. Removing baseURL lets better-auth use
window.location.origin automatically, which works correctly on any domain.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/auth-client.ts

@@ -7,6 +7,5 @@ import { genericOAuthClient } from "better-auth/client/plugins";
  * Auth client configuration
  */
 export const authClient = createAuthClient({
-  baseURL: process.env.NEXT_PUBLIC_APP_URL,
   plugins: [genericOAuthClient()],
 });