feat(stammbaum): serialise pan/zoom state to URL params (#692)

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

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

@@ -2,6 +2,7 @@ import { describe, it, expect } from 'vitest';
 import {
 	clampZoom,
 	parsePanZoomParams,
+	serializePanZoomParams,
 	DEFAULT_VIEW,
 	DEFAULT_ZOOM,
 	MIN_ZOOM,
@@ -59,3 +60,18 @@ describe('parsePanZoomParams', () => {
 		expect(parsePanZoomParams({ z: '-3' }).z).toBe(MIN_ZOOM);
 	});
 });
+
+describe('serializePanZoomParams', () => {
+	it('produces string cx/cy/z keys', () => {
+		expect(serializePanZoomParams({ x: 120, y: -40, z: 1.5 })).toEqual({
+			cx: '120',
+			cy: '-40',
+			z: '1.5'
+		});
+	});
+
+	it('round-trips through parsePanZoomParams', () => {
+		const state = { x: 87.5, y: -12.25, z: 2.4 };
+		expect(parsePanZoomParams(serializePanZoomParams(state))).toEqual(state);
+	});
+});

frontend/src/lib/person/genealogy/panZoom.ts

@@ -56,3 +56,8 @@ export function parsePanZoomParams(raw: {
 		z: clampZoom(finiteOr(raw.z, DEFAULT_ZOOM))
 	};
 }
+
+/** Serialise a view state into URL query params (the inverse of {@link parsePanZoomParams}). */
+export function serializePanZoomParams(state: PanZoomState): { cx: string; cy: string; z: string } {
+	return { cx: String(state.x), cy: String(state.y), z: String(state.z) };
+}