fix(scripts): headless check now says 'run npm run build' instead of failing obscurely

Without dist/ it reported 'no webgl context captured', which points at WebGL
when the actual problem was that there was no page to load.

scripts/headless-check.mjs

@@ -23,6 +23,11 @@ const dist = join(root, 'dist');
 const PORT = Number(process.env.CHECK_PORT || 4176);
 const CDP_PORT = Number(process.env.CDP_PORT || 9333);
 
+if (!existsSync(join(dist, 'index.html'))) {
+  console.error('HEADLESS CHECK FAILED: dist/index.html not found. Run `npm run build` first.');
+  process.exit(1);
+}
+
 function findChrome() {
   if (process.env.CHROME) return process.env.CHROME;
   const candidates = [