Rolls the standalone "Date issues" toggle back into FlagFilter as a
third value ('any' | 'discarded' | 'date_warning') so the date-warning
control lives in the same popover as Discarded, where operators expect
all flag-style filters. Drops the redundant dateWarning boolean and
its URL param.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>