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/adapters/workStateNormalization.ts

@@ -86,7 +86,11 @@ function normalizeBackgroundStatus(
   const raw = typeof value === "string" ? value.toLowerCase() : "";
   if (raw === "completed" || raw === "success" || raw === "succeeded") return "completed";
   if (raw === "failed" || raw === "failure") return "failed";
+  if (raw === "cancelfailed" || raw === "cancel_failed" || raw === "cancel-failed") {
+    return "cancelFailed";
+  }
   if (raw === "cancelled" || raw === "canceled" || raw === "expired") return "cancelled";
+  if (raw === "cancelling" || raw === "canceling") return "cancelling";
   if (raw === "running" || raw === "started") return "running";
   if (raw === "delivered") return "delivered";
   // Backend `queued`/`waiting` collapse to the cancellable "pending" bucket.