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/layout.test.ts

@@ -117,14 +117,25 @@ describe("MockLayoutGateway", () => {
     expect(activeId).toBe(newActive);
   });
 
-  it("setActiveLayout switches the active layout", async () => {
+  it("setActiveLayout switches the active layout and returns the authoritative id", async () => {
     const gw = new MockLayoutGateway();
     const { layoutId } = await gw.createLayout("p1", "Alt");
-    await gw.setActiveLayout("p1", layoutId);
+    const result = await gw.setActiveLayout("p1", layoutId);
+    expect(result.activeId).toBe(layoutId);
     const { activeId } = await gw.listLayouts("p1");
     expect(activeId).toBe(layoutId);
   });
 
+  it("setActiveLayout self-heals: a stale id keeps (and returns) the current active id", async () => {
+    const gw = new MockLayoutGateway();
+    const { activeId: initial } = await gw.listLayouts("p1");
+    const result = await gw.setActiveLayout("p1", "ghost-layout-id");
+    // The requested id does not exist → active id is unchanged and returned.
+    expect(result.activeId).toBe(initial);
+    const { activeId } = await gw.listLayouts("p1");
+    expect(activeId).toBe(initial);
+  });
+
   it("loadLayout with a specific layoutId loads that layout's tree", async () => {
     const gw = new MockLayoutGateway();
     const { layoutId } = await gw.createLayout("p1", "Named");