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/taskCancellation.test.ts

@@ -0,0 +1,24 @@
+import { describe, expect, it } from "vitest";
+
+import { canCancelTask } from "./taskCancellation";
+import type { TaskStatus } from "./types";
+
+describe("canCancelTask", () => {
+  it.each<TaskStatus>(["queued", "assigned", "dispatched"])(
+    "allows cancelling a %s task when the caller can submit",
+    (status) => {
+      expect(canCancelTask(status, true)).toBe(true);
+    },
+  );
+
+  it.each<TaskStatus>(["done", "failed", "cancelled"])(
+    "refuses to cancel a terminal %s task even when the caller can submit",
+    (status) => {
+      expect(canCancelTask(status, true)).toBe(false);
+    },
+  );
+
+  it("refuses to cancel a cancellable task when the caller lacks submit permission", () => {
+    expect(canCancelTask("assigned", false)).toBe(false);
+  });
+});