feat(stammbaum): sync view to shareable ?cx&cy&z URL (#692)

A view-keyed effect mirrors pan/zoom into the URL via replaceState (URL read
untracked to avoid a feedback loop). State survives panel open/close
(US-PANEL-002 AC1) and a shared link reproduces the view (AC2).

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

frontend/src/lib/person/genealogy/StammbaumTree.svelte.test.ts

@@ -1,6 +1,7 @@
 import { describe, it, expect, vi } from 'vitest';
 import { render } from 'vitest-browser-svelte';
 import StammbaumTree from './StammbaumTree.svelte';
+import type { PanZoomState } from './panZoom';
 
 const ID_A = '00000000-0000-0000-0000-000000000001';
 const ID_B = '00000000-0000-0000-0000-000000000002';
@@ -512,7 +513,7 @@ describe('StammbaumTree node rendering branches', () => {
 });
 
 describe('StammbaumTree keyboard pan/zoom (#692)', () => {
-	const renderTree = (onPanZoom: ReturnType<typeof vi.fn>) =>
+	const renderTree = (onPanZoom: (state: PanZoomState) => void) =>
 		render(StammbaumTree, {
 			nodes: [{ id: ID_A, displayName: 'Anna', familyMember: true }],
 			edges: [],