Bulk update modal: cannot clear rating/tags/acquired-on (null/[]/'' overloading unchanged vs clear)

[mforce]

ACKNOWLEDGED codex review comment on PR #161 (CollectionList.tsx:271/272). The modal overloads null / [] / '' as both 'leave unchanged' and 'clear', and the hasBulkEdit guard omits those keys — so personalRating: null, clearing tags, and clearing acquisition dates (all API-supported via BulkUpdates) are not reachable from the UI. Needs a tri-state/touched-flags design so clear operations can be submitted. The server side is correct; this is a client UX gap.