fix: Guard against a missing toolbar in the fragment jumper

Companion to upstream's def7144, which guards the same script against a
missing article.doc. Upstream needs no toolbar guard because every one of its
layouts has one; our landing layout does not, so jumpToAnchor could throw on
a page that has an article but no toolbar.

Treats an absent toolbar as a zero offset rather than returning early, so
anchor links keep working instead of silently doing nothing.

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

Author: lfrancke

ui/src/js/03-fragment-jumper.js

@@ -20,7 +20,7 @@
       window.location.hash = '#' + this.id
       e.preventDefault()
     }
-    var y = computePosition(this, 0) - toolbar.getBoundingClientRect().bottom
+    var y = computePosition(this, 0) - (toolbar ? toolbar.getBoundingClientRect().bottom : 0)
     var instant = e === false && supportsScrollToOptions
     instant ? window.scrollTo({ left: 0, top: y, behavior: 'instant' }) : window.scrollTo(0, y)
   }