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/views/calendar.js

@@ -1,7 +1,8 @@
 // Calendar grid for the trip's date range. Real weeks as rows (Mon–Sun
 // columns); days outside the range are greyed. Each in-range day shows its
 // entries as compact, type-coloured chips. Clicking a day opens the editor.
-import { el } from '../dom.js';
+import { el, toast } from '../dom.js';
+import { api } from '../api.js';
 import {
   ENTRY_TYPES,
   typeInfo,
@@ -68,9 +69,14 @@ export function renderCalendar(tctx) {
     { class: 'card calendar-section' },
     el(
       'div',
-      { class: 'section-head' },
-      el('h2', {}, 'Calendar'),
-      el('p', { class: 'muted' }, 'Click a day to add or edit entries.'),
+      { class: 'section-head cal-section-head' },
+      el(
+        'div',
+        {},
+        el('h2', {}, 'Calendar'),
+        el('p', { class: 'muted' }, 'Click a day to add or edit entries.'),
+      ),
+      syncTransportsButton(tctx),
     ),
     legend(),
   );
@@ -97,7 +103,7 @@ export function renderCalendar(tctx) {
   for (const week of weeks) {
     // Each week is a stay-bands strip (all-day bars) above a row of day cells.
     const weekEl = el('div', { class: 'cal-week' });
-    const bands = renderWeekBands(week.map(ymd), stayLayout, tctx);
+    const bands = renderWeekBands(week.map(ymd), stayLayout, entriesById, tctx);
     if (bands) weekEl.appendChild(bands);
     const daysRow = el('div', { class: 'cal-week-days' });
     for (const date of week) {
@@ -111,12 +117,39 @@ export function renderCalendar(tctx) {
   // which bubbles up here — sweep any lingering drop highlight.
   cal.addEventListener('dragend', () => {
     for (const n of cal.querySelectorAll('.cal-day-drop')) n.classList.remove('cal-day-drop');
+    for (const n of cal.querySelectorAll('.cal-bands-drop')) n.classList.remove('cal-bands-drop');
   });
 
   section.appendChild(cal);
   return section;
 }
 
+// "↻ Sync transports" — reconciles auto-created transport entries after stays
+// have been dragged/reshuffled (see docs/API.md, "Regenerating auto-transports").
+function syncTransportsButton(tctx) {
+  const btn = el('button', { class: 'btn btn-sm btn-ghost', type: 'button' }, '↻ Sync transports');
+  btn.addEventListener('click', async () => {
+    btn.disabled = true;
+    const original = btn.textContent;
+    btn.textContent = 'Syncing…';
+    try {
+      const res = await api.trips.regenerateTransports(tctx.tripId);
+      const parts = [];
+      if (res.created) parts.push(`${res.created} added`);
+      if (res.updated) parts.push(`${res.updated} updated`);
+      if (res.deleted) parts.push(`${res.deleted} removed`);
+      toast(parts.length ? `Transports synced: ${parts.join(', ')}` : 'Transports already in sync', 'success');
+      await tctx.refreshTrip();
+    } catch (err) {
+      toast(err.message);
+    } finally {
+      btn.disabled = false;
+      btn.textContent = original;
+    }
+  });
+  return btn;
+}
+
 function dayCell(date, rangeStart, rangeEnd, byDate, contByDate, dropoffByDate, entriesById, tctx, currency) {
   const key = ymd(date);
   const inRange = date >= rangeStart && date <= rangeEnd;