feat(nav): add BackButton component calling history.back()

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

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

@@ -0,0 +1,29 @@
+import { describe, it, expect, afterEach, vi } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { page } from 'vitest/browser';
+import BackButton from './BackButton.svelte';
+
+afterEach(cleanup);
+
+describe('BackButton', () => {
+	it('renders a button with "Zurück" text', async () => {
+		render(BackButton);
+		await expect.element(page.getByRole('button', { name: /zurück/i })).toBeInTheDocument();
+	});
+
+	it('calls history.back() when clicked', async () => {
+		const backSpy = vi.spyOn(history, 'back').mockImplementation(() => {});
+		render(BackButton);
+
+		await page.getByRole('button', { name: /zurück/i }).click();
+
+		expect(backSpy).toHaveBeenCalledOnce();
+		backSpy.mockRestore();
+	});
+
+	it('has min-h-[44px] for touch target compliance', async () => {
+		render(BackButton);
+		const btn = document.querySelector('button');
+		expect(btn?.className).toContain('min-h-[44px]');
+	});
+});