Add drag & drop reordering of trips on the dashboard

- trip_members.sort_order (per-user, migrated) drives GET /api/trips order
- PATCH /api/trips/:id/order updates only the caller's membership row
- Trip cards get a drag handle reusing the generalized row-reorder helper;
  clicking the card still navigates

public/js/views/dragdrop.js

@@ -1,8 +1,10 @@
-// HTML5 drag-and-drop helpers, no libraries. Two flows:
+// 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).
 //   • Day editor — drag a row's handle to reorder entries within one day.
-// Touch devices don't fire HTML5 DnD events, so both flows are desktop-only and
+//   • Trip dashboard — drag a card's handle to reorder trips (shares the day
+//     editor's row-reorder machinery, see enableReorder below).
+// Touch devices don't fire HTML5 DnD events, so all flows are desktop-only and
 // degrade to the existing click/edit behaviour with no polyfill.
 import { api } from '../api.js';
 import { toast } from '../dom.js';
@@ -62,24 +64,26 @@ export function makeDayDropTarget(cell, targetDate, targetCount, entriesById, tc
   });
 }
 
-// ---------- Day editor: reorder entries within a day ----------
+// ---------- Shared reorder machinery ----------
 
-// `rows`: [{ node, handle, entry }] in current display order. Dragging a row's
+// `rows`: [{ node, handle, item }] in current display order. Dragging a row's
 // handle over another row shows a top/bottom insertion indicator; on drop the
-// new order is computed and sort_order PATCHed for ONLY the entries whose index
-// changed, then `refresh()` re-renders (the editor stays open on the same day).
-export function enableRowReorder(rows, refresh) {
+// new order is computed and `commitPatch(id, newSortOrder)` is awaited for ONLY
+// the items whose index changed, then `refresh()` re-renders. `getId`/`getSort`
+// read the identifying fields off `item` (entries use id/sort_order, trips too,
+// but kept generic so callers don't need matching shapes).
+function enableReorder(rows, getId, getSort, commitPatch, refresh) {
   let draggingId = null;
 
   const clearAll = () => {
     for (const r of rows) r.node.classList.remove('drop-before', 'drop-after', 'dragging');
   };
 
-  for (const { node, handle, entry } of rows) {
+  for (const { node, handle, item } of rows) {
     handle.setAttribute('draggable', 'true');
     handle.addEventListener('dragstart', (e) => {
-      draggingId = entry.id;
-      e.dataTransfer.setData(DND_MIME, String(entry.id));
+      draggingId = getId(item);
+      e.dataTransfer.setData(DND_MIME, String(draggingId));
       e.dataTransfer.effectAllowed = 'move';
       e.dataTransfer.setDragImage(node, 0, 0);
       node.classList.add('dragging');
@@ -87,7 +91,7 @@ export function enableRowReorder(rows, refresh) {
     handle.addEventListener('dragend', () => { draggingId = null; clearAll(); });
 
     node.addEventListener('dragover', (e) => {
-      if (draggingId == null || entry.id === draggingId) return;
+      if (draggingId == null || getId(item) === draggingId) return;
       e.preventDefault();
       e.dataTransfer.dropEffect = 'move';
       const rect = node.getBoundingClientRect();
@@ -99,26 +103,26 @@ export function enableRowReorder(rows, refresh) {
       if (!node.contains(e.relatedTarget)) node.classList.remove('drop-before', 'drop-after');
     });
     node.addEventListener('drop', async (e) => {
-      if (draggingId == null || entry.id === draggingId) return;
+      if (draggingId == null || getId(item) === draggingId) return;
       e.preventDefault();
       const after = node.classList.contains('drop-after');
       const movedId = draggingId;
       clearAll();
       draggingId = null;
-      await commitReorder(rows, movedId, entry.id, after, refresh);
+      await commitReorder(rows, movedId, getId(item), after, getId, getSort, commitPatch, refresh);
     });
   }
 }
 
-async function commitReorder(rows, movedId, targetId, after, refresh) {
-  const moved = rows.find((r) => r.entry.id === movedId).entry;
-  const ordered = rows.map((r) => r.entry).filter((e) => e.id !== movedId);
-  const targetIndex = ordered.findIndex((e) => e.id === targetId);
+async function commitReorder(rows, movedId, targetId, after, getId, getSort, commitPatch, refresh) {
+  const moved = rows.find((r) => getId(r.item) === movedId).item;
+  const ordered = rows.map((r) => r.item).filter((it) => getId(it) !== movedId);
+  const targetIndex = ordered.findIndex((it) => getId(it) === targetId);
   ordered.splice(after ? targetIndex + 1 : targetIndex, 0, moved);
   try {
     for (let i = 0; i < ordered.length; i++) {
-      if (ordered[i].sort_order !== i) {
-        await api.entries.update(ordered[i].id, { sort_order: i });
+      if (getSort(ordered[i]) !== i) {
+        await commitPatch(getId(ordered[i]), i);
       }
     }
     await refresh();
@@ -126,3 +130,30 @@ async function commitReorder(rows, movedId, targetId, after, refresh) {
     toast(err.message);
   }
 }
+
+// ---------- Day editor: reorder entries within a day ----------
+
+// `rows`: [{ node, handle, entry }] — see enableReorder above for the mechanics.
+export function enableRowReorder(rows, refresh) {
+  enableReorder(
+    rows.map((r) => ({ node: r.node, handle: r.handle, item: r.entry })),
+    (entry) => entry.id,
+    (entry) => entry.sort_order,
+    (id, sort_order) => api.entries.update(id, { sort_order }),
+    refresh,
+  );
+}
+
+// ---------- Trip dashboard: reorder trip cards ----------
+
+// `rows`: [{ node, handle, trip }] — same mechanics as enableRowReorder, but
+// PATCHes the caller's per-user `trip_members.sort_order` via api.trips.reorder.
+export function enableTripReorder(rows, refresh) {
+  enableReorder(
+    rows.map((r) => ({ node: r.node, handle: r.handle, item: r.trip })),
+    (trip) => trip.id,
+    (trip) => trip.sort_order,
+    (id, sort_order) => api.trips.reorder(id, sort_order),
+    refresh,
+  );
+}