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.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) };
+}