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/app-tauri/src/commands.rs

@@ -4447,8 +4447,7 @@ pub async fn spawn_background_command(
 
 /// `cancel_background_task` — request cancellation of a running background task.
 ///
-/// The terminal `Cancelled` state is written by the completion sink; this returns
-/// the task as currently persisted (absent if it no longer exists).
+/// Returns the task as currently persisted after the cancel request.
 ///
 /// # Errors
 /// Returns an [`ErrorDto`] (`INVALID` for a malformed id, `PROCESS`/`STORE` on
@@ -4457,13 +4456,13 @@ pub async fn spawn_background_command(
 pub async fn cancel_background_task(
     task_id: String,
     state: State<'_, AppState>,
-) -> Result<Option<BackgroundTaskDto>, ErrorDto> {
+) -> Result<BackgroundTaskDto, ErrorDto> {
     let id = parse_task_id(&task_id)?;
     state
         .cancel_background_task
         .execute(id)
         .await
-        .map(|out| out.task.map(BackgroundTaskDto::from))
+        .map(|out| BackgroundTaskDto::from(out.task))
         .map_err(ErrorDto::from)
 }