feat(background-task): rendu live des tâches de fond — subscriber UI + canal IPC (#58)

Câble un canal attachable au flux de sortie d'une tâche de fond (runner
infrastructure + commande app-tauri + port/adaptateurs frontend) et le
panneau ProjectWorkStatePanel s'y abonne pour un rendu live au lieu d'un
état figé au dernier snapshot.

Validations obtenues avant commit :
- cargo test -p infrastructure --test background_task_runner : vert
- cargo check -p backend -p app-tauri : vert
- npx vitest run src/features/workstate/workstate.test.tsx : vert
- npm run typecheck : vert
- verdict QA #58 : vert

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/adapters/http/requestResponseGateways.ts

@@ -71,8 +71,10 @@ import type {
   SkillGateway,
   TemplateGateway,
   WorkStateGateway,
+  BackgroundTaskAttachment,
 } from "@/ports";
 import { normalizeProjectWorkState } from "../workStateNormalization";
+import { unsupportedOnWeb } from "./unsupported";
 import { normalizeTurnPage } from "../conversationNormalization";
 import { normalizeProfileModelCatalog } from "../profileCatalog";
 import type { HttpInvoker } from "./httpInvoker";
@@ -443,6 +445,12 @@ export class HttpWorkStateGateway implements WorkStateGateway {
     const state = await this.http.invoke<unknown>("get_project_work_state", { projectId });
     return normalizeProjectWorkState(state);
   }
+  async attachBackgroundTask(
+    _taskId: string,
+    _onData: (bytes: Uint8Array) => void,
+  ): Promise<BackgroundTaskAttachment> {
+    return unsupportedOnWeb("Attaching to a background task live stream");
+  }
   async cancelBackgroundTask(taskId: string): Promise<void> {
     await this.http.invoke<unknown>("cancel_background_task", { taskId });
   }