feat(stammbaum): round pan/zoom URL params for readable shared links (#692)

Pan rounded to 2 decimals, zoom to 3, so ?cx/?cy/?z no longer carry float
noise like cx=457.8300882631206.

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

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

@@ -71,9 +71,18 @@ export function parsePanZoomParams(raw: {
 	};
 }
 
-/** Serialise a view state into URL query params (the inverse of {@link parsePanZoomParams}). */
+/** Format a number with at most `dp` decimals, dropping trailing zeros. */
+function round(n: number, dp: number): string {
+	return String(Number(n.toFixed(dp)));
+}
+
+/**
+ * Serialise a view state into URL query params (the inverse of
+ * {@link parsePanZoomParams}). Pan is rounded to 2 decimals and zoom to 3 so
+ * shared links stay readable (no `cx=457.8300882631206` float noise).
+ */
 export function serializePanZoomParams(state: PanZoomState): { cx: string; cy: string; z: string } {
-	return { cx: String(state.x), cy: String(state.y), z: String(state.z) };
+	return { cx: round(state.x, 2), cy: round(state.y, 2), z: round(state.z, 3) };
 }
 
 /**