Fix doubled border below search bar

The search bar had its own border-b which stacked with the header's
border-b once the category tabs row was removed. Drop the redundant
one from the search bar.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

layouts/partials/header.html

@@ -32,7 +32,7 @@
   </div>
 
   {{/* ── search bar ── */}}
-  <div class="relative flex items-stretch border-t border-[var(--rule-2)] border-b border-[var(--rule)]
+  <div class="relative flex items-stretch border-t border-[var(--rule-2)]
               [background:color-mix(in_oklab,var(--paper)_95%,transparent)]">
     <label class="flex-1 flex items-center gap-3 px-[var(--pad)] py-[10px] border-r border-[var(--rule-2)] min-w-0 cursor-text"
            for="searchInput">