Persist sidebar collapsed preference to localStorage

frontend/src/layouts/__tests__/MainLayout.test.tsx

@@ -18,7 +18,7 @@ import { MainLayout } from "../../layouts/MainLayout";
 // Mocks
 // ---------------------------------------------------------------------------
 
-vi.mock("../../providers/AuthProvider", () => ({
+vi.mock("../../hooks/useAuth", () => ({
   useAuth: () => ({ logout: vi.fn() }),
 }));
 
@@ -56,6 +56,7 @@ function renderLayout(): void {
 describe("MainLayout", () => {
   beforeEach(() => {
     vi.clearAllMocks();
+    localStorage.clear();
   });
 
   it("renders the navigation sidebar", () => {
@@ -75,4 +76,17 @@ describe("MainLayout", () => {
     await userEvent.click(toggleButton);
     expect(screen.queryByText("BanGUI")).not.toBeInTheDocument();
   });
+
+  it("initialises sidebar collapsed state from localStorage", () => {
+    localStorage.setItem("bangui_sidebar_collapsed", "true");
+    renderLayout();
+    expect(screen.queryByText("BanGUI")).not.toBeInTheDocument();
+  });
+
+  it("persists sidebar collapsed state to localStorage when toggled", async () => {
+    renderLayout();
+    const toggleButton = screen.getByRole("button", { name: /collapse sidebar/i });
+    await userEvent.click(toggleButton);
+    expect(localStorage.getItem("bangui_sidebar_collapsed")).toBe("true");
+  });
 });