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/trip/list.html

@@ -64,6 +64,13 @@
     {% endfor %}
     <div>Total CO₂: {{ "{:,.1f}".format(total_co2_kg / 1000.0) }} tonnes</div>
 
+    {% for transport_type, co2_kg in co2_by_transport_type %}
+      <div>
+        {{ transport_type | title }}
+        CO₂: {{ "{:,.1f}".format(co2_kg) }} kg
+      </div>
+    {% endfor %}
+
     {% for trip in items %}
       {{ trip_item(trip) }}
     {% endfor %}