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/adapters/mock/index.ts

@@ -846,11 +846,17 @@ export class MockLayoutGateway implements LayoutGateway {
     return { activeId: ps.activeId };
   }
 
-  async setActiveLayout(projectId: string, layoutId: string): Promise<void> {
+  async setActiveLayout(
+    projectId: string,
+    layoutId: string,
+  ): Promise<{ activeId: string }> {
     const ps = this.getProjectLayouts(projectId);
+    // Self-heal (I4): only adopt the requested id when it exists; otherwise keep
+    // the current active id. Either way the returned id is authoritative.
     if (ps.layouts.some((l) => l.id === layoutId)) {
       ps.activeId = layoutId;
     }
+    return { activeId: ps.activeId };
   }
 }