Add new country count display to trip statistics

Show the number of new countries visited each year in parentheses next to the total country count.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

templates/trip/stats.html

@@ -26,6 +26,15 @@
     <div>Trips in {{ year }}: {{ year_stats.count }}</div>
     <div>Conferences in {{ year }}: {{ year_stats.conferences }}</div>
     <div>{{ countries | count }} countries visited in {{ year }}:
+      {% set new_countries = [] %}
+      {% for c in countries %}
+        {% if c.alpha_2 not in previously_visited %}
+          {% set _ = new_countries.append(c) %}
+        {% endif %}
+      {% endfor %}
+      {% if new_countries %}
+        ({{ new_countries | count }} new)
+      {% endif %}
       {% for c in countries %}
         <span class="text-nowrap border border-2 px-2 my-3 mx-1">
           {{ c.flag }} {{ c.name }} ({{ c.alpha_2 }})