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>

agenda/types.py

@@ -230,6 +230,21 @@ class Trip:
 
         return list(transport_distances.items())
 
+    def co2_by_transport_type(self) -> list[tuple[str, float]]:
+        """Calculate the total CO₂ emissions for each type of transport.
+
+        Any travel item with a missing or None 'co2_kg' field is ignored.
+        """
+        transport_co2: defaultdict[str, float] = defaultdict(float)
+
+        for item in self.travel:
+            co2_kg = item.get("co2_kg")
+            if co2_kg:
+                transport_type: str = item.get("type", "unknown")
+                transport_co2[transport_type] += float(co2_kg)
+
+        return list(transport_co2.items())
+
     def elements(self) -> list[TripElement]:
         """Trip elements ordered by time."""
         elements: list[TripElement] = []

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>

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 %}

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 %}