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>

.gitignore

@@ -20,6 +20,3 @@ Desktop.ini
 
 # Package lock file
 package-lock.json
-
-# Generated preview data (run: cd tools && go run . preview all)
-preview/rendered.js