Replace the two most error-prone positional admin commands with Block Kit
modals (override and holiday-add) opened from a new App Home admin section,
while keeping the typed subcommands as a fallback. Validation and side
effects are factored into shared helpers so the modal and command paths
can't drift, and every action/view handler re-checks is_admin against
get_admin_users() so a modal opened from Home can't bypass authorization.
Adds Schedule.list_overrides for the upcoming-overrides overview.
Refs: #136
