Add optional checklist dueAt with the shared selected-calendar picker and badge. Return checklist and item deadlines from checklist and single-card GETs. Accept validated timezone-qualified ISO deadlines in checklist/item POST and PUT; null clears, omission preserves. Keep existing write permissions and ownership checks. Node regression and Chromium save/clear test pass.
Thanks to rmb82 and xet7 !
Fixes#6693,