fix: handle absolute nav links without prepending collection base URL

Nav links starting with '/' are already absolute site paths. The old
code unconditionally prepended nav_base (also starting with '/'),
producing a double-prefix like /openvox/latest//openvox-server/latest/...
and causing a 404.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Michael Harp <mike@mikeharp.com>

Author: miharp

_includes/nav-item.html

@@ -28,7 +28,16 @@ <h2 class="text">{{ include.item.text }}</h2>
     </section>
   </div>
 {% elsif include.item.link %}
-  {% assign nav_item_url = include.item.link | prepend: nav_base %}
+  {%- comment -%}
+    Links starting with '/' are already absolute site paths. Prepending nav_base (which also
+    starts with '/') would produce a broken double-prefix like /openvox/latest//other/page.html.
+  {%- endcomment -%}
+  {% assign nav_link_first_char = include.item.link | slice: 0 %}
+  {% if nav_link_first_char == '/' %}
+    {% assign nav_item_url = include.item.link %}
+  {% else %}
+    {% assign nav_item_url = include.item.link | prepend: nav_base %}
+  {% endif %}
 
   <div class="VPSidebarItem level-{{ include.level }} is-link{% if nav_item_url == page.url %} is-active{% endif %}">
     <div class="item">