test(nav): add ThemeToggle spec covering label derivation in both modes

Covers the concern raised during PR review: themeLabel $derived logic
had no tests. Adds 4 tests — aria-label and title=aria-label assertions
in light mode and dark mode — mirroring the NotificationBell pattern.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/ThemeToggle.svelte.spec.ts

@@ -0,0 +1,45 @@
+import { afterEach, beforeEach, describe, expect, it } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { page } from 'vitest/browser';
+import ThemeToggle from './ThemeToggle.svelte';
+
+afterEach(() => {
+	cleanup();
+	localStorage.removeItem('theme');
+});
+
+describe('ThemeToggle — label derivation (light mode)', () => {
+	beforeEach(() => {
+		localStorage.setItem('theme', 'light');
+	});
+
+	it('aria-label invites switching to dark mode when theme is light', async () => {
+		render(ThemeToggle);
+		const btn = await page.getByRole('button').element();
+		expect(btn.getAttribute('aria-label')).toBe('Zu dunklem Design wechseln');
+	});
+
+	it('title equals aria-label in light mode', async () => {
+		render(ThemeToggle);
+		const btn = await page.getByRole('button').element();
+		expect(btn.getAttribute('title')).toBe(btn.getAttribute('aria-label'));
+	});
+});
+
+describe('ThemeToggle — label derivation (dark mode)', () => {
+	beforeEach(() => {
+		localStorage.setItem('theme', 'dark');
+	});
+
+	it('aria-label invites switching to light mode when theme is dark', async () => {
+		render(ThemeToggle);
+		const btn = await page.getByRole('button').element();
+		expect(btn.getAttribute('aria-label')).toBe('Zu hellem Design wechseln');
+	});
+
+	it('title equals aria-label in dark mode', async () => {
+		render(ThemeToggle);
+		const btn = await page.getByRole('button').element();
+		expect(btn.getAttribute('title')).toBe(btn.getAttribute('aria-label'));
+	});
+});