Add pagination to category search for large categories

Large categories like "Living people" (900k+ articles) were impractical
because the code tried to download all members before displaying results.
Now stops after collecting ~200 articles and provides a "Next page" link.

Also fixes the MediaWiki API continuation protocol: passes the full
continue dict (not just gcmcontinue) so imcontinue responses are handled
properly, and reduces gcmlimit from "max" to 50 so each batch's images
fit in one API response.

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

templates/category.html

@@ -44,6 +44,12 @@
         {% endfor %}
       </div>
 
+      {% if gcmcontinue %}
+      <div class="mt-3">
+        <a href="{{ url_for('category_search', cat=cat, gcmcontinue=gcmcontinue) }}" class="btn btn-outline-primary">Next page &raquo;</a>
+      </div>
+      {% endif %}
+
       {% else %}
       <div class="alert alert-success mt-3">
         All articles in this category have images!