fix(preview): ship rendered.js, hide loader on missing file

- Remove preview/rendered.js from .gitignore so the project works
  out-of-the-box after clone (no Go required for static preview).
- Hide loading spinner when rendered.js is not found; previously
  the spinner stayed visible forever because render() returned
  early when 'rendered' was null.
- Regenerate rendered.js with all current template fixes.

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

preview/index.html

@@ -297,6 +297,7 @@ document.addEventListener('keydown',e=>{
 
     document.getElementById('loading').style.display='none';
   } else {
+    document.getElementById('loading').style.display='none';
     showMessage('rendered.js not found — run <code>cd tools &amp;&amp; go run . preview all</code> to generate', 'error');
   }
   render();