fix issue

src/main.js

@@ -1,9 +1,13 @@
-import { mount } from 'svelte'
 import './app.css'
-import App from './App.svelte'
 
-const app = mount(App, {
-  target: document.getElementById('app'),
-})
+// Only load and mount the app in a real browser (document + mount available).
+// Avoids "mount() is not available on the server" when the bundle runs in SSR/workers.
+let app = null
+const target = typeof document !== 'undefined' && document.getElementById('app')
+if (target) {
+  const { mount } = await import('svelte')
+  const { default: App } = await import('./App.svelte')
+  app = mount(App, { target })
+}
 
 export default app

vite.config.js

@@ -5,8 +5,8 @@ import { svelte } from '@sveltejs/vite-plugin-svelte'
 export default defineConfig(({ mode }) => ({
   plugins: [svelte()],
   resolve: {
-    // Use browser Svelte build in tests so mount() is available (avoid "lifecycle_function_unavailable")
-    conditions: mode === 'test' ? ['browser'] : [],
+    // Prefer browser Svelte build so mount() is available (avoid "lifecycle_function_unavailable" on server/workers)
+    conditions: ['browser'],
   },
   test: {
     include: ['src/**/*.{test,spec}.{js,ts}'],