weirsoe-ui.js: publish measured fixed-navbar height as --wt-nav-height

So content can clear a fixed top navbar exactly instead of guessing (CSS guesses
overlapped on mobile when the bar was taller). Measured on load/resize/orientation
from .navbar.is-fixed-top (or [data-wt-fixed-nav]); consumers use
padding-top: calc(var(--wt-nav-height) + gap).

weirsoe-ui.js

@@ -182,6 +182,17 @@
     document.querySelectorAll('[data-wt-sortable]').forEach(initSortable);
   }
 
+  // Publish the real fixed-navbar height as --wt-nav-height so content can clear
+  // it exactly (CSS guesses overlap when the navbar is taller than assumed).
+  function setNavHeight() {
+    var nav = document.querySelector('[data-wt-fixed-nav], .navbar.is-fixed-top');
+    if (nav) document.documentElement.style.setProperty('--wt-nav-height', nav.offsetHeight + 'px');
+  }
+  setNavHeight();
+  window.addEventListener('load', setNavHeight);
+  window.addEventListener('resize', setNavHeight);
+  window.addEventListener('orientationchange', setNavHeight);
+
   document.addEventListener('click', function (e) {
     var btn = e.target.closest && e.target.closest('[data-copy-link]');
     if (!btn) return;