fix(preview): stop loader re-appearing when rendered.js is missing

When rendered is null, render() was setting loading.style.display='flex'
before returning — overriding the hide already done in the init error
branch. Now render() silently returns, leaving init to handle the error.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

preview/index.html

@@ -183,7 +183,7 @@ function transform(html, mode) {
 function render() {
   const frame = document.getElementById('preview-frame');
   const loading = document.getElementById('loading');
-  if(!rendered) { loading.style.display='flex'; frame.style.display='none'; return; }
+  if(!rendered) return; // no data yet — init handles the error display
 
   frame.style.display = 'none';
   loading.style.display = 'flex';