fix(runtime): cancel de background task immédiat et fiable depuis le workstate (#199, QA verte)

Transforme l'annulation d'une background task en contrat immédiat, observable et non-best-effort:

- domain/background_task: états Cancelling/Cancelled/CancelFailed et transitions idempotentes; un kill PTY qui échoue est surfacé en CancelFailed plutôt qu'en succès silencieux.

- infrastructure/runner + store: cancel effectif de la task ciblée, reconcile marque Cancelling même sans live handle, et départage CancelFailed des tâches pending au boot.

- application/background + workstate + orchestrator/wake: chemin de cancel initié depuis le workstate, projection synchronisée de l'état (pas d'attente d'un cycle ambigu).

- frontend: projection workstate de l'état cancelled/cancelling/cancelFailed (cancellingRow/cancelFailedRow/cancelButtons), normalization et services.

Tests QA verts: background_task_cancel, background_task_runner cancel_wins_even_if_output_is_still_available, background_task_store reconcile_marks_cancelling_without_live_handle_cancel_failed_and_pending, workstate.test.tsx.

frontend/src/features/web/WebWorkspace.tsx

@@ -507,17 +507,23 @@ function AgentLiveRow({
 
 const TASK_STATUS_LABEL: Record<BackgroundCompletion["status"], string> = {
   running: "Running",
+  cancelling: "Cancelling",
   completed: "Completed",
   failed: "Failed",
+  cancelFailed: "Cancel failed",
   cancelled: "Cancelled",
   pending: "Pending",
   delivered: "Delivered",
 };
 
 function taskStatusClass(status: BackgroundCompletion["status"]): string {
-  if (status === "running" || status === "pending") return "bg-warning/15 text-warning";
+  if (status === "running" || status === "pending" || status === "cancelling") {
+    return "bg-warning/15 text-warning";
+  }
   if (status === "completed" || status === "delivered") return "bg-success/10 text-success";
-  if (status === "failed" || status === "cancelled") return "bg-danger/10 text-danger";
+  if (status === "failed" || status === "cancelFailed" || status === "cancelled") {
+    return "bg-danger/10 text-danger";
+  }
   return "bg-raised text-muted";
 }