Add client-side search with / shortcut

Lazy-loads /index.json on first keystroke, fuzzy-matches titles and
descriptions, keyboard-navigable results (arrows, Enter, Esc).
Based on https://gist.github.com/cmod/5410eae147e4318164258742dd053993

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

themes/hugo-theme-ipng/layouts/partials/header.html

@@ -3,7 +3,7 @@
     <div class="myname"> 
         <h2><a href="{{ default .Site.Home .Site.BaseURL }}">{{ default .Site.Params.Author .Site.Params.siteHeading }}</a></h2> 
     </div>
-    <nav>
+    <nav id="mainNav">
         <ul class="navbar">
         {{- /* info about current page */ -}}
             {{- $currentPage := . -}}
@@ -67,4 +67,8 @@
         {{- end }}
         </ul>
     </nav>
+    <div id="fastSearch">
+        <input id="searchInput" tabindex="0" placeholder="Search…" aria-label="Search" autocomplete="off">
+        <ul id="searchResults"></ul>
+    </div>
 </header>