feat(stammbaum): first-load touch affordance hint (#692)

Add StammbaumAffordance: a touch-only "drag to explore · pinch to zoom" hint
that auto-dismisses on the first canvas pointer interaction (wired via the
gesture action's onGestureStart) or the explicit close, and stays dismissed for
30 days via a localStorage timestamp (boolean gate only, never rendered).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/person/genealogy/StammbaumAffordance.svelte.test.ts

@@ -0,0 +1,34 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+import { render } from 'vitest-browser-svelte';
+import StammbaumAffordance from './StammbaumAffordance.svelte';
+
+const STORAGE_KEY = 'stammbaumAffordanceDismissedAt';
+
+describe('StammbaumAffordance (#692)', () => {
+	beforeEach(() => localStorage.clear());
+
+	it('shows the hint on a touch device that has not dismissed it', async () => {
+		render(StammbaumAffordance, { touch: true });
+		await vi.waitFor(() => expect(document.querySelector('[role="status"]')).not.toBeNull());
+		expect(document.body.textContent).toContain('Ziehen');
+	});
+
+	it('does not show on non-touch devices (OQ-008)', async () => {
+		render(StammbaumAffordance, { touch: false });
+		expect(document.querySelector('[role="status"]')).toBeNull();
+	});
+
+	it('hides and records dismissal when the close button is clicked', async () => {
+		render(StammbaumAffordance, { touch: true });
+		const dismiss = [...document.querySelectorAll<HTMLButtonElement>('button')][0];
+		dismiss.click();
+		await vi.waitFor(() => expect(document.querySelector('[role="status"]')).toBeNull());
+		expect(localStorage.getItem(STORAGE_KEY)).toBeTruthy();
+	});
+
+	it('does not reappear within the 30-day window (NFR-USE-001)', async () => {
+		localStorage.setItem(STORAGE_KEY, String(Date.now()));
+		render(StammbaumAffordance, { touch: true });
+		expect(document.querySelector('[role="status"]')).toBeNull();
+	});
+});