test: make tests less fragile

tests/unit/components/layout/AppHeader.test.ts

@@ -1,6 +1,6 @@
 import AppHeader from "$components/layout/AppHeader.svelte";
 import { themeState } from "$states/theme.svelte";
-import { fireEvent, render } from "@testing-library/svelte";
+import { fireEvent, render, screen } from "@testing-library/svelte";
 import { beforeEach, describe, expect, it } from "vitest";
 
 describe("AppHeader.svelte", () => {
@@ -9,10 +9,9 @@ describe("AppHeader.svelte", () => {
   });
 
   it("should toggle theme on button click", async () => {
-    const { container } = render(AppHeader);
+    render(AppHeader);
 
-    const toggleButton = container.querySelector("button");
-    if (!toggleButton) throw new Error("Toggle button not found");
+    const toggleButton = screen.getByRole("button", { name: /toggle theme/i });
 
     themeState.theme = "dark";
     await fireEvent.click(toggleButton);