Add transport sync button and allow dropping stays onto band-covered days

- Auto-created transports carry auto_ref {from,to} stay ids; new
  POST /api/trips/:id/transports/regenerate reconciles them against the
  current stay order (re-date/re-title kept bridges preserving mode and
  price, delete orphans, create missing) without touching manual
  transports or flight-covered gaps
- Sync transports button in the calendar header with result toast
- The week band strip is now a drop target resolving the day from the
  pointer position, so stays can be dropped onto spots covered by other
  stays; overlapping stays stack in band lanes

public/js/api.js

@@ -71,6 +71,7 @@ export const api = {
     removeMember: (id, userId) => del(`/api/trips/${id}/members/${userId}`),
     route: (id) => get(`/api/trips/${id}/route`),
     costs: (id) => get(`/api/trips/${id}/costs`),
+    regenerateTransports: (id) => post(`/api/trips/${id}/transports/regenerate`, {}),
   },
   entries: {
     create: (tripId, payload) => post(`/api/trips/${tripId}/entries`, payload),