fix: restore search by keeping hidden search-button element in DOM

The just-the-docs.js initNav() calls jtd.addEvent on the #search-button
element unconditionally. When the element was removed (to avoid overlap
with the OSA chat widget), jtd.addEvent(null, ...) threw an error,
preventing initSearch() from ever executing. The search index was never
built, leaving the header search input non-functional.

Restore the button with display:none so the JS doesn't crash while
keeping it invisible to avoid overlap with the chat widget.

Author: neuromechanist

_layouts/default.html

@@ -189,6 +189,12 @@ <h2 class="text-delta">Table of contents</h2>
     </div>
 
     {% if site.search_enabled != false %}
+      {% if site.search.button %}
+        <a href="#" id="search-button" class="search-button" style="display:none;">
+          <svg viewBox="0 0 24 24" class="icon"><use xlink:href="#svg-search"></use></svg>
+        </a>
+      {% endif %}
+
       <div class="search-overlay"></div>
     {% endif %}
   </div>