feat(stammbaum): animate fit-to-screen, snap under reduced motion (#692)

Fit-to-screen tweens to the default view over 300ms via animateView (eased,
lerpView-driven) and snaps instantly when prefers-reduced-motion is set
(US-PAN-004 AC2, NFR-A11Y-003).

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

frontend/src/lib/person/genealogy/animateView.test.ts

@@ -0,0 +1,17 @@
+import { describe, it, expect, vi } from 'vitest';
+import { animateView } from './animateView';
+
+describe('animateView (reduced motion)', () => {
+	const from = { x: 0, y: 0, z: 1 };
+	const to = { x: 80, y: -30, z: 2 };
+
+	it('snaps straight to the target in a single frame when reduced motion is on', () => {
+		const onFrame = vi.fn();
+		const cancel = animateView(from, to, onFrame, { reducedMotion: true });
+
+		expect(onFrame).toHaveBeenCalledTimes(1);
+		expect(onFrame).toHaveBeenCalledWith(to);
+		expect(typeof cancel).toBe('function');
+		cancel();
+	});
+});