Fix tab group spacing: make document-tabs block and use inline margin

Custom elements are display:inline by default, so margin-top on the
inner tablist div was not producing visible spacing. Move the margin
to the document-tabs element itself with an inline style (immune to
Tailwind recompile timing) and add class="block" so it participates
in the block flow and the margin takes effect.

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

app/views/submit_form/show.html.erb

@@ -65,9 +65,9 @@
            n_pages > 0 && pages_text.size >= n_pages
          end %>
       <% if has_full_text %>
-        <document-tabs>
+        <document-tabs class="block" style="margin-top: 4rem">
           <div role="tablist" aria-label="<%= t('document_view_options') %>"
-               class="flex border-b border-base-300 mb-2 mt-16">
+               class="flex border-b border-base-300 mb-2">
             <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">