fix(nav): keep aux nav links single-line on narrow mobile widths (#519)

Code review of #518 found that on narrow viewports (e.g. ~280px, the
Galaxy Z Fold cover screen) or with a longer/additional aux_links
entry, the aux nav button text wraps to a second line. That grows
#main-header taller than the padding-top reserved on .site-nav, so the
fixed search box ends up overlapping the first nav item's click target
by a few pixels.

Force aux nav links to stay on a single line and scroll horizontally
instead, using the theme's own existing overflow-x: auto, so
#main-header's height (and the matching reserved padding-top) stays
constant regardless of viewport width or aux_links content.

Verified via headless Chromium at 280/320/375px widths and with a
stress-tested 4th, longer aux link: no overlap in any case.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: gkorland

_includes/head_custom.html

@@ -42,6 +42,14 @@
   .site-nav.nav-open {
     padding-top: 7.5rem;
   }
+
+  /* Keep aux nav links on a single line (scrolling horizontally via the
+     theme's existing overflow-x: auto) instead of wrapping. Otherwise
+     #main-header grows taller on narrow screens or with more aux_links,
+     and the padding-top reserved above would no longer match its height. */
+  .aux-nav .site-button {
+    white-space: nowrap;
+  }
 }
 </style>