test(LanguageSwitcher): add Vitest unit tests for inverted prop

Covers active/inactive class tokens for both inverted=true and inverted=false,
and verifies setLocale is called with the correct locale on button click.

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

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

@@ -0,0 +1,94 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { page } from 'vitest/browser';
+import LanguageSwitcher from './LanguageSwitcher.svelte';
+
+const mockSetLocale = vi.hoisted(() => vi.fn());
+
+vi.mock('$lib/paraglide/runtime', () => ({
+	getLocale: vi.fn(() => 'de'),
+	setLocale: mockSetLocale
+}));
+
+beforeEach(() => mockSetLocale.mockClear());
+afterEach(cleanup);
+
+// ─── inverted=true (dark background) ──────────────────────────────────────
+
+describe('LanguageSwitcher – inverted=true', () => {
+	it('active locale button has text-white and font-bold', async () => {
+		render(LanguageSwitcher, { inverted: true });
+
+		const el = await page.getByRole('button', { name: 'DE' }).element();
+
+		expect(el.className).toMatch(/\btext-white\b/);
+		expect(el.className).toMatch(/\bfont-bold\b/);
+	});
+
+	it('inactive locale buttons have text-white/70', async () => {
+		render(LanguageSwitcher, { inverted: true });
+
+		const el = await page.getByRole('button', { name: 'EN' }).element();
+
+		expect(el.className).toMatch(/text-white\/70/);
+	});
+
+	it('inactive locale buttons do not have font-bold', async () => {
+		render(LanguageSwitcher, { inverted: true });
+
+		const el = await page.getByRole('button', { name: 'EN' }).element();
+
+		expect(el.className).not.toMatch(/\bfont-bold\b/);
+	});
+});
+
+// ─── inverted=false (light background) ─────────────────────────────────────
+
+describe('LanguageSwitcher – inverted=false', () => {
+	it('active locale button has text-ink and font-bold', async () => {
+		render(LanguageSwitcher, { inverted: false });
+
+		const el = await page.getByRole('button', { name: 'DE' }).element();
+
+		expect(el.className).toMatch(/\btext-ink\b/);
+		expect(el.className).toMatch(/\bfont-bold\b/);
+	});
+
+	it('inactive locale buttons have text-ink-3', async () => {
+		render(LanguageSwitcher, { inverted: false });
+
+		const el = await page.getByRole('button', { name: 'EN' }).element();
+
+		expect(el.className).toMatch(/\btext-ink-3\b/);
+	});
+
+	it('inactive locale buttons do not have text-white', async () => {
+		render(LanguageSwitcher, { inverted: false });
+
+		const el = await page.getByRole('button', { name: 'EN' }).element();
+
+		expect(el.className).not.toMatch(/\btext-white\b/);
+	});
+});
+
+// ─── locale switching ──────────────────────────────────────────────────────
+
+describe('LanguageSwitcher – locale switching', () => {
+	it('calls setLocale with en when EN button is clicked', async () => {
+		render(LanguageSwitcher, { inverted: false });
+
+		const el = await page.getByRole('button', { name: 'EN' }).element();
+		el.click();
+
+		expect(mockSetLocale).toHaveBeenCalledWith('en');
+	});
+
+	it('calls setLocale with es when ES button is clicked', async () => {
+		render(LanguageSwitcher, { inverted: false });
+
+		const el = await page.getByRole('button', { name: 'ES' }).element();
+		el.click();
+
+		expect(mockSetLocale).toHaveBeenCalledWith('es');
+	});
+});