BlocksScreen couldn't load/save blocks containing Artist or
MultiCollection items (creatable upstream; ReplaceBlockItemsHandler
already supports both) - itemFromResponse dropped multiCollectionId
and left selectionName empty for those kinds, and toRequestItem
hardcoded multiCollectionId to null, so re-saving such a block
produced a 422 and made it uneditable. Carry the ids/names through
DraftItem and render them read-only (parity with Blazor, which
doesn't offer these kinds in the creation picker either).
Also key the item AsyncPicker by selection identity + collection
type so switching rows or collection type remounts it with clean
state, instead of carrying over a stale query/options list that
could store a wrong-kind id.
timothy
a20fef6096