Update trip statistics CO2 display with transport breakdown

- Change label from "Flight CO₂" to "Total CO₂" for accuracy
- Add CO₂ breakdown by transport type (flight/train/ferry)
- Show values in kg for <1000kg, tonnes for ≥1000kg
- Track CO₂ emissions by transport type in stats calculation

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

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

agenda/stats.py

@@ -10,8 +10,12 @@ def travel_legs(trip: Trip, stats: StrDict) -> None:
     """Calculate stats for travel legs."""
     for leg in trip.travel:
         stats.setdefault("co2_kg", 0)
+        stats.setdefault("co2_by_transport_type", {})
         if "co2_kg" in leg:
             stats["co2_kg"] += leg["co2_kg"]
+            transport_type = leg["type"]
+            stats["co2_by_transport_type"].setdefault(transport_type, 0)
+            stats["co2_by_transport_type"][transport_type] += leg["co2_kg"]
         if leg["type"] == "flight":
             stats.setdefault("flight_count", 0)
             stats.setdefault("airlines", Counter())

templates/trip/stats.html

@@ -45,7 +45,25 @@
       {% endif %}
     </div>
     {% if year_stats.co2_kg %}
-      <div>Flight CO₂: {{ "{:,.1f}".format(year_stats.co2_kg / 1000.0) }} tonnes</div>
+      <div>Total CO₂: 
+        {% if year_stats.co2_kg >= 1000 %}
+          {{ "{:,.2f}".format(year_stats.co2_kg / 1000.0) }} tonnes
+        {% else %}
+          {{ "{:,.0f}".format(year_stats.co2_kg) }} kg
+        {% endif %}
+      </div>
+      {% if year_stats.co2_by_transport_type %}
+        {% for transport_type, co2_kg in year_stats.co2_by_transport_type.items() %}
+          <div style="margin-left: 20px;">
+            {{ transport_type | title }} CO₂: 
+            {% if co2_kg >= 1000 %}
+              {{ "{:,.2f}".format(co2_kg / 1000.0) }} tonnes
+            {% else %}
+              {{ "{:,.0f}".format(co2_kg) }} kg
+            {% endif %}
+          </div>
+        {% endfor %}
+      {% endif %}
     {% endif %}
     <div>Trains segments in {{ year }}: {{ year_stats.train_count or 0 }}</div>
     <div>Total distance in {{ year}}: {{ format_distance(year_stats.total_distance or 0) }}</div>