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

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

frontend/src/lib/components/BackButton.svelte

@@ -0,0 +1,25 @@
+<script lang="ts">
+import { m } from '$lib/paraglide/messages.js';
+</script>
+
+<button
+	type="button"
+	onclick={() => history.back()}
+	class="group mb-4 inline-flex min-h-[44px] items-center text-xs font-bold tracking-widest text-ink-2 uppercase transition-colors outline-none hover:text-ink focus-visible:ring-2 focus-visible:ring-focus-ring"
+>
+	<svg
+		class="mr-2 h-4 w-4 transform transition-transform group-hover:-translate-x-1"
+		fill="none"
+		stroke="currentColor"
+		viewBox="0 0 24 24"
+		aria-hidden="true"
+	>
+		<path
+			stroke-linecap="round"
+			stroke-linejoin="round"
+			stroke-width="2"
+			d="M10 19l-7-7m0 0l7-7m-7 7h18"
+		/>
+	</svg>
+	{m.btn_back()}
+</button>

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]');
+	});
+});