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_page.html

@@ -78,6 +78,8 @@
 {% set end = trip.end %}
 {% set total_distance = trip.total_distance() %}
 {% set distances_by_transport_type = trip.distances_by_transport_type() %}
+{% set total_co2_kg = trip.total_co2_kg() %}
+{% set co2_by_transport_type = trip.co2_by_transport_type() %}
 
 {% block content %}
 <div class="row">
@@ -112,6 +114,18 @@
       {% endfor %}
     {% endif %}
 
+    {% if total_co2_kg %}
+      <div>Total CO₂: {{ "{:,.1f}".format(total_co2_kg) }} kg</div>
+    {% endif %}
+
+    {% if co2_by_transport_type %}
+      {% for transport_type, co2_kg in co2_by_transport_type %}
+        <div>{{ transport_type | title }} CO₂:
+          {{ "{:,.1f}".format(co2_kg) }} kg
+        </div>
+      {% endfor %}
+    {% endif %}
+
 
     {% set delta = human_readable_delta(trip.start) %}
     {% if delta %}