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/tests/workstate.rs

@@ -568,9 +568,15 @@ fn background_task(
             .unwrap()
             .transition(BackgroundTaskState::Waiting, created_at_ms + 20)
             .unwrap(),
+        BackgroundTaskState::Cancelling => base
+            .transition(BackgroundTaskState::Running, created_at_ms + 10)
+            .unwrap()
+            .transition(BackgroundTaskState::Cancelling, created_at_ms + 20)
+            .unwrap(),
         BackgroundTaskState::Completed
         | BackgroundTaskState::Failed
         | BackgroundTaskState::Cancelled
+        | BackgroundTaskState::CancelFailed
         | BackgroundTaskState::Expired => {
             let result = match state {
                 BackgroundTaskState::Completed => BackgroundTaskResult::Success {
@@ -591,13 +597,18 @@ fn background_task(
                     finished_at_ms: created_at_ms + 20,
                     reason: "cancelled".to_owned(),
                 },
+                BackgroundTaskState::CancelFailed => BackgroundTaskResult::CancelFailed {
+                    finished_at_ms: created_at_ms + 20,
+                    error: "cancel failed".to_owned(),
+                },
                 BackgroundTaskState::Expired => BackgroundTaskResult::Expired {
                     finished_at_ms: created_at_ms + 20,
                     reason: "expired".to_owned(),
                 },
                 BackgroundTaskState::Queued
                 | BackgroundTaskState::Running
-                | BackgroundTaskState::Waiting => unreachable!(),
+                | BackgroundTaskState::Waiting
+                | BackgroundTaskState::Cancelling => unreachable!(),
             };
             let running = base
                 .transition(BackgroundTaskState::Running, created_at_ms + 10)