Add CO2 breakdown by transport type to trip list and detail pages

Implements bug #194 by adding CO2 emission display by transport type:
- Add co2_by_transport_type() method to Trip class
- Display CO2 breakdown on trip list page and individual trip items
- Display CO2 breakdown on individual trip detail pages
- Show both total CO2 and breakdown by transport type (flight/train/ferry)

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

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

templates/macros.html

@@ -351,6 +351,16 @@
       <div>Total CO₂: {{ "{:,.1f}".format(total_co2_kg) }} kg</div>
     {% endif %}
 
+    {% set co2_by_transport = trip.co2_by_transport_type() %}
+    {% if co2_by_transport %}
+      {% for transport_type, co2_kg in co2_by_transport %}
+        <div>
+          {{ transport_type | title }}
+          CO₂: {{ "{:,.1f}".format(co2_kg) }} kg
+        </div>
+      {% endfor %}
+    {% endif %}
+
     {% if trip.schengen_compliance %}
       <div>
         <strong>Schengen:</strong>