- Widen TaskStatus to include "cancelled"; add it to TasksView's
STATUSES filter dropdown.
- Add TaskCancellationResponse type and cancelTask(taskId) to api.ts.
- Add a Cancel button to TasksView's task detail panel, gated on
tasks:submit and a non-terminal task status; updates the displayed
status on success and surfaces errors via the existing error path.
- Extract the cancellability rule into a pure taskCancellation.ts
module (mirroring taskProgress.ts/plannerHistory.ts) with unit
tests, since the project has no Vue component-mounting test setup.
Task 6/9 of task-cancellation change.
