Log a clear console error when the cookie banner is missing

Silently returning made a missing banner.html integration
indistinguishable from a working page, which cost real debugging
time. Fail loud instead, pointing at the README.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

assets/js/hugo_cookie_consent/cookie_consent.js

@@ -1,7 +1,14 @@
 (function () {
   var KEY = 'cookie_consent';
   var banner = document.getElementById('cookie-banner');
-  if (!banner) return;
+  if (!banner) {
+    console.error(
+      '[hugo_cookie_consent] #cookie-banner not found in the DOM. ' +
+      'Did you include the hugo_cookie_consent/banner.html partial in your site\'s layout? ' +
+      'See the module README ("Installation in your Hugo site").'
+    );
+    return;
+  }
   updateStatus();
 
   var consent = localStorage.getItem(KEY);