Make stay bands draggable to move stays between days

Dragging any week-segment of a stay band onto an in-range day moves the
whole stay there, preserving its length (same one-PATCH date+end_date
shift the multi-day chips use). Clicking a band still opens the editor.

public/js/views/dragdrop.js

@@ -1,6 +1,7 @@
 // HTML5 drag-and-drop helpers, no libraries. Three flows:
-//   • Calendar — drag a day chip onto another in-range day to move that entry
-//     (shifts end_date by the same delta so multi-day spans stay intact).
+//   • Calendar — drag a day chip (or a stay area band) onto another in-range
+//     day to move that entry (shifts end_date by the same delta so multi-day
+//     spans / stay lengths stay intact).
 //   • Day editor — drag a row's handle to reorder entries within one day.
 //   • Trip dashboard — drag a card's handle to reorder trips (shares the day
 //     editor's row-reorder machinery, see enableReorder below).
@@ -30,6 +31,21 @@ export function makeChipDraggable(node, entry) {
   node.addEventListener('dragend', () => node.classList.remove('dragging'));
 }
 
+// Mark a stay area band segment as a drag source carrying its entry id. Same
+// mechanics as makeChipDraggable — dropping on an in-range day moves the
+// whole stay to start there, preserving its length (end_date shifts by the
+// same delta in makeDayDropTarget below).
+export function makeBandDraggable(node, entry) {
+  node.setAttribute('draggable', 'true');
+  node.classList.add('cal-band-draggable');
+  node.addEventListener('dragstart', (e) => {
+    e.dataTransfer.setData(DND_MIME, String(entry.id));
+    e.dataTransfer.effectAllowed = 'move';
+    node.classList.add('dragging');
+  });
+  node.addEventListener('dragend', () => node.classList.remove('dragging'));
+}
+
 // Wire an in-range day cell as a drop target. On drop the dragged entry moves to
 // `targetDate` with sort_order = `targetCount` (appended after that day's
 // entries); an entry with an end_date has it shifted by the same day-delta in