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

tests/api.test.js

@@ -325,7 +325,7 @@ test('entry CRUD and full-row shape', async () => {
   const entry = create.body.entry;
   assert.deepEqual(
     Object.keys(entry).sort(),
-    ['date', 'end_date', 'details', 'end_time', 'id', 'lat', 'lng', 'location_name', 'paid_by', 'participants', 'price', 'rental', 'segments', 'sort_order', 'split_mode', 'start_time', 'title', 'transport_mode', 'trip_id', 'type'].sort()
+    ['auto_ref', 'date', 'end_date', 'details', 'end_time', 'id', 'lat', 'lng', 'location_name', 'paid_by', 'participants', 'price', 'rental', 'segments', 'sort_order', 'split_mode', 'start_time', 'title', 'transport_mode', 'trip_id', 'type'].sort()
   );
   assert.equal(entry.end_date, null);
   assert.equal(entry.details, '');