Show exploration days before/after each conference on trip page

For each conference, calculate the number of free days available
before and after, relative to the trip boundary or adjacent
conference. Multi-conference trips show the gap between conferences
as "after" the first and "before" the second.

Closes #189

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

agenda/trip.py

@@ -456,6 +456,39 @@ def coordinate_dict(item: StrDict, coord_type: str) -> StrDict:
     }
 
 
+def conference_free_days(trip: Trip) -> dict[str, tuple[int, int]]:
+    """Return (days_before, days_after) exploration days for each conference.
+
+    Keyed by the conference start date as an ISO string. Days are relative to
+    the trip boundary or the adjacent conference's end/start for multi-conference
+    trips.
+    """
+    if not trip.conferences or not trip.end:
+        return {}
+
+    def conf_attend_start(c: StrDict) -> date:
+        return as_date(c.get("attend_start") or c["start"])
+
+    def conf_attend_end(c: StrDict) -> date:
+        return as_date(c.get("attend_end") or c["end"])
+
+    sorted_confs = sorted(trip.conferences, key=conf_attend_start)
+    result: dict[str, tuple[int, int]] = {}
+
+    for i, conf in enumerate(sorted_confs):
+        before_boundary = conf_attend_end(sorted_confs[i - 1]) if i > 0 else trip.start
+        after_boundary = (
+            conf_attend_start(sorted_confs[i + 1])
+            if i < len(sorted_confs) - 1
+            else trip.end
+        )
+        days_before = (conf_attend_start(conf) - before_boundary).days
+        days_after = (after_boundary - conf_attend_end(conf)).days
+        result[str(conf["start"])] = (days_before, days_after)
+
+    return result
+
+
 def collect_trip_coordinates(trip: Trip) -> list[StrDict]:
     """Extract and de-duplicate travel location coordinates from trip."""
     coords = []

templates/trip_page.html

@@ -239,6 +239,16 @@
                 {% elif item.price and item.currency %}
                   <span class="badge bg-info text-nowrap">price: {{ item.price }} {{ item.currency }}</span>
                 {% endif %}
+                {% set free_days = conference_free_days.get(item.start | string) %}
+                {% if free_days %}
+                  {% set days_before, days_after = free_days %}
+                  {% if days_before > 0 %}
+                    <span class="badge bg-secondary text-nowrap">{{ days_before }} day{{ 's' if days_before != 1 }} to explore before</span>
+                  {% endif %}
+                  {% if days_after > 0 %}
+                    <span class="badge bg-secondary text-nowrap">{{ days_after }} day{{ 's' if days_after != 1 }} to explore after</span>
+                  {% endif %}
+                {% endif %}
               </p>
             </div>
           </div>

web_view.py

@@ -1144,6 +1144,7 @@ def trip_page(start: str) -> str:
         destination_times=get_destination_timezones(trip),
         human_readable_delta=agenda.utils.human_readable_delta,
         trip_weather=trip_weather,
+        conference_free_days=agenda.trip.conference_free_days(trip),
     )