Move p4-loaded attribute setter out of P4.svelte

src/packager/P4.svelte

@@ -16,8 +16,6 @@
 
   let projectData;
 
-  document.body.setAttribute('p4-loaded', '');
-
   const darkMedia = window.matchMedia('(prefers-color-scheme: dark)');
   let systemTheme = darkMedia.matches ? 'dark' : 'light';
   if (darkMedia.addEventListener) {

src/packager/index.js

@@ -6,6 +6,8 @@ const app = new P4({
   target: document.getElementById('app')
 });
 
+document.body.setAttribute('p4-loaded', '');
+
 if (process.env.ENABLE_SERVICE_WORKER && 'serviceWorker' in navigator) {
   navigator.serviceWorker.register(serviceWorker)
     .then(() => {