Increase tab group top margin to 64px in signing form

The header has margin-bottom: -16px which pulls the next element up,
so mt-10 (40px) only produced ~24px of visible gap. mt-16 (64px) gives
~48px of actual space between the heading and the tab group.

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

app/views/submit_form/show.html.erb

@@ -67,7 +67,7 @@
       <% if has_full_text %>
         <document-tabs>
           <div role="tablist" aria-label="<%= t('document_view_options') %>"
-               class="flex border-b border-base-300 mb-2 mt-10">
+               class="flex border-b border-base-300 mb-2 mt-16">
             <button role="tab" id="tab-pdf" aria-selected="true"
                     aria-controls="panel-pdf" tabindex="0"
                     class="px-4 py-2 text-sm font-semibold border-b-2 -mb-px border-neutral text-base-content">