refactor: use Vite built-in DEV flag instead of VITE_OAUTH_ENABLED

import.meta.env.DEV is already true during `bun run dev` and false in
production builds, so a separate env var is unnecessary.

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

opal-web/src/lib/stores/auth.js

@@ -17,7 +17,7 @@ import { getItem, setItem, removeItem } from '$lib/utils/storage.js';
  */
 
 const STORAGE_KEY = 'opal_auth';
-const DEV_MODE = import.meta.env.VITE_OAUTH_ENABLED === 'false';
+const DEV_MODE = import.meta.env.DEV;
 
 /**
  * Load auth state from localStorage