feat(ui): add setting to keep the action bar visible while scrolling (#1095)

Adds a 'Keep Action Bar Visible' toggle (default off) under
Settings > Interface > Layout Settings. When enabled, the controls bar
(Refresh, Download, etc.) and the breadcrumb nav are wrapped in a shared
sticky container (.sticky-topbar) so both stay pinned as one unit; when
disabled, the wrapper is display: contents and the original behavior
(only the breadcrumb stays visible) is preserved.

templates/checkpoints.html

@@ -54,8 +54,10 @@
 {% endblock %}
 
 {% block content %}
+    <div class="sticky-topbar">
     {% include 'components/controls.html' %}
     {% include 'components/breadcrumb.html' %}
+    </div>
     {% include 'components/duplicates_banner.html' %}
     {% include 'components/folder_sidebar.html' %}