fix: navbar was position:relative (in flow), causing scroll-jump on posts

.navbar had a Bootstrap-leftover 'position: relative' that, on source order
(components load after the .fixed-top utility, equal specificity), overrode
.fixed-top's position:fixed. The fixed-top navbar was actually in flow, adding
~85px in the <header> above <main>; single.js's scroll handler then toggled it
in/out of flow (relative<->fixed) near the top, jumping the content. Removed the
redundant position so .fixed-top wins: header collapses to 0, navbar overlays,
content clears it via the 6.5rem margin. Fixes posts and tightens pages too.

css/styles.css

@@ -260,7 +260,9 @@ body { color: #232323; }
   padding-left: 0;
 }
 .navbar {
-  position: relative;
+  /* no position here — the header navbar is always .fixed-top (position:fixed);
+     a `position: relative` would win on source order and drop it into flow,
+     adding ~85px above the content and causing a scroll-jump (single.js). */
   display: flex;
   flex-wrap: wrap;
   align-items: center;