Add dark mode support with persisted OS-aware theme selection

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

@@ -89,4 +89,9 @@ describe("MainLayout", () => {
     await userEvent.click(toggleButton);
     expect(localStorage.getItem("bangui_sidebar_collapsed")).toBe("true");
   });
+
+  it("renders a theme toggle button in the sidebar footer", () => {
+    renderLayout();
+    expect(screen.getByRole("button", { name: /switch to dark mode/i })).toBeInTheDocument();
+  });
 });