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/routes/stammbaum/+page.svelte

@@ -10,6 +10,7 @@ import {
 	clampZoom,
 	ZOOM_STEP_KB
 } from '$lib/person/genealogy/panZoom';
+import { animateView, prefersReducedMotion } from '$lib/person/genealogy/animateView';
 import type { components } from '$lib/generated/api';
 
 type PersonNodeDTO = components['schemas']['PersonNodeDTO'];
@@ -37,8 +38,12 @@ function zoomIn() {
 function zoomOut() {
 	view = { ...view, z: clampZoom(view.z - ZOOM_STEP_KB) };
 }
+let cancelAnimation = () => {};
 function fitToScreen() {
-	view = DEFAULT_VIEW;
+	cancelAnimation();
+	cancelAnimation = animateView(view, DEFAULT_VIEW, (v) => (view = v), {
+		reducedMotion: prefersReducedMotion()
+	});
 }
 </script>