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/db/database.js

@@ -556,6 +556,19 @@ function initDb() {
         _db.prepare("INSERT OR IGNORE INTO addons (id, name, description, type, icon, enabled, sort_order) VALUES ('collab', 'Collab', 'Notes, polls, and live chat for trip collaboration', 'trip', 'Users', 0, 6)").run();
       } catch {}
     },
+    // 26: Per-assignment times (instead of shared place times)
+    () => {
+      try { _db.exec('ALTER TABLE day_assignments ADD COLUMN assignment_time TEXT'); } catch {}
+      try { _db.exec('ALTER TABLE day_assignments ADD COLUMN assignment_end_time TEXT'); } catch {}
+      // Copy existing place times to assignments as initial values
+      try {
+        _db.exec(`
+          UPDATE day_assignments SET
+            assignment_time = (SELECT place_time FROM places WHERE places.id = day_assignments.place_id),
+            assignment_end_time = (SELECT end_time FROM places WHERE places.id = day_assignments.place_id)
+        `);
+      } catch {}
+    },
     // Future migrations go here (append only, never reorder)
   ];