feat(mobile): show last commit date in expand row on mobile

Add an expand-commit span inside the expand row that displays the last
commit date. Hidden on desktop, visible only on mobile (max-width: 960px)
via media query, mirroring the commit column that appears in the full
table. Relative time formatting is applied via JS on page load.

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

website/templates/index.html

@@ -220,6 +220,7 @@
                   rel="noopener"
                   >{{ entry.url | replace("https://", "") }}</a
                 >
+                {% if entry.last_commit_at %}<span class="expand-commit"><span class="expand-sep">/</span>Last commit <time datetime="{{ entry.last_commit_at }}">{{ entry.last_commit_at[:10] }}</time></span>{% endif %}
               </div>
             </div>
           </td>