Make reordering work on the page you are actually looking at

Two defects, both found by driving the real editor in Firefox:

1. The editor drew grab handles with a grab cursor on sets it refuses to
   reorder, so the live Food menu looked draggable and did nothing. Handles and
   the move buttons are now rendered only when reordering is possible, and the
   notice says what can still be done.

2. Reordering was saved only from dragend. Browsers suppress mouse events during
   a drag and dragend is not guaranteed to arrive, which left the rows visibly
   reordered while the server kept the old order — the drag looked like it
   worked and then reverted on reload. The order now commits from dragover after
   a pause in the drag, with drop/dragend/mouse events as faster paths, deduped
   against the last confirmed order and reported in the page.

To make (1) coherent the Current set may now be reordered while its Pages stay
fixed: order is presentation, media URLs are content-hashed and pages are
rendered per request, so nothing stale can be served (ADR 0006). Archives stay
frozen. Admin pages are no-store and assets cache for 5 minutes, so a fix like
this one is not masked by a stale admin.js.

scripts/check-dnd.mjs drives the real editor and asserts the drag persists; it
is the only automated check that reaches this path.

docs/adr/0006-reordering-the-current-set.md

@@ -0,0 +1,5 @@
+# The Current set can be reordered, though its Pages are fixed
+
+A published Menu Set's Pages are still fixed, but the order of the Current set can be changed, because order is presentation rather than content: Viewing image URLs are content-hashed, pages are rendered per request, and nothing downstream caches a rendered page, so moving a Page cannot serve anything stale. A restaurant that finds its live menu in the wrong order fixes it in seconds instead of re-uploading twenty photos; Archives stay frozen as historical records.
+
+**Considered Options**: duplicating the Current set into a Draft to reorder it. That keeps one simple rule but makes the commonest correction the most expensive, and it moves the menu's URL for what is only a change of order.