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

server/src/routes/days.js

@@ -13,7 +13,9 @@ function getAssignmentsForDay(dayId) {
   const assignments = db.prepare(`
     SELECT da.*, p.id as place_id, p.name as place_name, p.description as place_description,
       p.lat, p.lng, p.address, p.category_id, p.price, p.currency as place_currency,
-      p.place_time, p.end_time, p.duration_minutes, p.notes as place_notes,
+      COALESCE(da.assignment_time, p.place_time) as place_time,
+      COALESCE(da.assignment_end_time, p.end_time) as end_time,
+      p.duration_minutes, p.notes as place_notes,
       p.image_url, p.transport_mode, p.google_place_id, p.website, p.phone,
       c.name as category_name, c.color as category_color, c.icon as category_icon
     FROM day_assignments da
@@ -89,7 +91,9 @@ router.get('/', authenticate, (req, res) => {
   const allAssignments = db.prepare(`
     SELECT da.*, p.id as place_id, p.name as place_name, p.description as place_description,
       p.lat, p.lng, p.address, p.category_id, p.price, p.currency as place_currency,
-      p.place_time, p.end_time, p.duration_minutes, p.notes as place_notes,
+      COALESCE(da.assignment_time, p.place_time) as place_time,
+      COALESCE(da.assignment_end_time, p.end_time) as end_time,
+      p.duration_minutes, p.notes as place_notes,
       p.image_url, p.transport_mode, p.google_place_id, p.website, p.phone,
       c.name as category_name, c.color as category_color, c.icon as category_icon
     FROM day_assignments da