Per-assignment times, participant avatar fix, UI improvements

- Times are now per-assignment instead of per-place, so the same place
  on different days can have different times
- Migration 26 adds assignment_time/assignment_end_time columns
- New endpoint PUT /assignments/:id/time for updating assignment times
- Time picker removed from place creation (only shown when editing)
- End-before-start validation disables save button
- Time collision warning shows overlapping activities on the same day
- Fix participant avatars using avatar_url instead of avatar filename
- Rename "Add Place" to "Add Place/Activity" (DE + EN)
- Improve README update instructions with docker inspect tip

client/src/api/client.js

@@ -97,6 +97,7 @@ export const assignmentsApi = {
   update: (tripId, dayId, id, data) => apiClient.put(`/trips/${tripId}/days/${dayId}/assignments/${id}`, data).then(r => r.data),
   getParticipants: (tripId, id) => apiClient.get(`/trips/${tripId}/assignments/${id}/participants`).then(r => r.data),
   setParticipants: (tripId, id, userIds) => apiClient.put(`/trips/${tripId}/assignments/${id}/participants`, { user_ids: userIds }).then(r => r.data),
+  updateTime: (tripId, id, times) => apiClient.put(`/trips/${tripId}/assignments/${id}/time`, times).then(r => r.data),
 }
 
 export const packingApi = {