fix(layout): auto-réparer l'onglet actif

stale

Le backend renvoie l'id actif autoritaire et le
    frontend l'adopte pour éviter de rejouer un layout
    disparu après overwrite externe.

Co-Authored-By: Claude Opus 4.8
    <noreply@anthropic.com>

frontend/src/ports/index.ts

@@ -375,8 +375,14 @@ export interface LayoutGateway {
   renameLayout(projectId: string, layoutId: string, name: string): Promise<void>;
   /** Deletes a layout; returns the new active layout id. */
   deleteLayout(projectId: string, layoutId: string): Promise<{ activeId: string }>;
-  /** Sets the active layout for a project. */
-  setActiveLayout(projectId: string, layoutId: string): Promise<void>;
+  /**
+   * Sets the active layout for a project. Returns the layout id the backend
+   * *actually* activated — authoritative (invariant I4): it equals the requested
+   * id when valid, else the unchanged current active id (self-healing fallback
+   * when the requested id was stale, e.g. after an external overwrite of
+   * `layouts.json`). Callers must adopt this id rather than the one they asked for.
+   */
+  setActiveLayout(projectId: string, layoutId: string): Promise<{ activeId: string }>;
 }
 
 /** Git: status/commit/checkout/… (L8). */