The toggle handler required onSubtaskUpdate callback to make the API
call, but TaskSubtasksCard never provided it. Extract a shared handler
that calls toggleTaskCompletion for persisted subtasks regardless of
whether onSubtaskUpdate is provided, falling back to updating local
state via onSubtasksChange.
When toggling subtask completion in TaskSubtasksSection, the toggle handler
required onSubtaskUpdate callback to make the API call. Components like
TaskSubtasksCard don't pass this callback, so toggling fell through to the
local-only state update path, never sending a network request.
Add an intermediate code path: when the subtask is persisted but
onSubtaskUpdate is not provided, call toggleTaskCompletion() directly
to persist the change, then update local state.
Fixes#920
