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.

crates/application/src/plugin/mod.rs

@@ -1940,9 +1940,11 @@ fn background_task_state_label(state: BackgroundTaskState) -> &'static str {
         BackgroundTaskState::Queued => "queued",
         BackgroundTaskState::Running => "running",
         BackgroundTaskState::Waiting => "waiting",
+        BackgroundTaskState::Cancelling => "cancelling",
         BackgroundTaskState::Completed => "completed",
         BackgroundTaskState::Failed => "failed",
         BackgroundTaskState::Cancelled => "cancelled",
+        BackgroundTaskState::CancelFailed => "cancelFailed",
         BackgroundTaskState::Expired => "expired",
     }
 }