fix(web): route la famille layout dans /api/invoke + fallback UX restauration — #212

Le web-server ne routait pas load_layout (UNKNOWN_COMMAND a l'ouverture
d'un projet web PC). Alignement de l'adapter web sur les contrats Tauri:
load_layout, mutate_layout, list_layouts, create_layout, rename_layout,
delete_layout, set_active_layout. useLayout ne propage plus l'erreur
brute: message utilisateur + fallback singleLeafTree.

frontend/src/features/layout/LayoutGrid.test.tsx

@@ -35,6 +35,31 @@ function renderGrid(layout: MockLayoutGateway, projectId = "p1") {
 }
 
 describe("LayoutGrid (with MockLayoutGateway)", () => {
+  it("falls back to an empty workspace with a user-facing message when layout restore fails", async () => {
+    const layout = new MockLayoutGateway();
+    vi.spyOn(layout, "loadLayout").mockRejectedValueOnce(
+      new Error("unknown command: load_layout"),
+    );
+    const warn = vi.spyOn(console, "warn").mockImplementation(() => {});
+
+    renderGrid(layout);
+
+    const alert = await screen.findByRole("alert");
+    expect(alert.textContent).toContain(
+      "Impossible de restaurer l’espace de travail de ce projet.",
+    );
+    expect(alert.textContent).not.toContain("unknown command");
+    await waitFor(() => {
+      expect(screen.getAllByTestId("layout-leaf")).toHaveLength(1);
+    });
+    expect(warn).toHaveBeenCalledWith(
+      "Layout restore failed",
+      expect.objectContaining({ message: "unknown command: load_layout" }),
+    );
+
+    warn.mockRestore();
+  });
+
   it("renders a single-leaf layout without throwing", async () => {
     const layout = new MockLayoutGateway();
     expect(() => renderGrid(layout)).not.toThrow();