Add Cloud Console cancel action and cancelled status

- 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.

cloud-console/src/types.ts

@@ -3,7 +3,8 @@ export type TaskStatus =
   | "assigned"
   | "dispatched"
   | "done"
-  | "failed";
+  | "failed"
+  | "cancelled";
 
 export interface TaskListItem {
   id: string;
@@ -41,6 +42,11 @@ export interface TaskListResponse {
   offset: number;
 }
 
+export interface TaskCancellationResponse {
+  task_id: string;
+  status: TaskStatus;
+}
+
 export interface TaskAttempt {
   task_id: string;
   attempt: number;