fix(stammbaum): URL pan/zoom sync never fired — gate replaceState on router-ready (#692)

replaceState throws 'before the router is initialized' during hydration, which
killed the sync $effect on its first tick so the URL never updated on pan/zoom.
Gate the write behind a flag flipped after the first post-mount tick() (router
started) plus a defensive try/catch. Verified live: zoom now updates ?z=.
The prior component test mocked replaceState and masked this.

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

frontend/src/routes/stammbaum/+page.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-import { untrack, tick } from 'svelte';
+import { untrack, tick, onMount } from 'svelte';
 import { m } from '$lib/paraglide/messages.js';
 import { page } from '$app/state';
 import { replaceState } from '$app/navigation';
@@ -62,18 +62,33 @@ function fitToScreen() {
 	});
 }
 
-// Mirror the view into shareable ?cx&cy&z params (OQ-003). Only `view` is a
+// SvelteKit's replaceState throws "before the router is initialized" if called
-// tracked dependency; the current URL is read untracked so the replaceState
+// during hydration (the router sets `started = true` only after onMount + the
-// write does not retrigger the effect. The state thus survives panel open/close
+// first effect tick). Gate the URL sync on a flag flipped after the first
-// (US-PANEL-002 AC1) and a shared link reproduces it (AC2).
+// post-mount tick() — which resolves once hydration is complete — so the write
+// only ever runs against a ready router.
+let routerReady = $state(false);
+onMount(() => {
+	tick().then(() => (routerReady = true));
+});
+
+// Mirror the view into shareable ?cx&cy&z params (OQ-003). Only `view` and
+// `routerReady` are tracked; the current URL is read untracked so the
+// replaceState write does not retrigger the effect. The state thus survives
+// panel open/close (US-PANEL-002 AC1) and a shared link reproduces it (AC2).
 $effect(() => {
 	const { cx, cy, z } = serializePanZoomParams(view);
+	if (!routerReady) return;
 	untrack(() => {
 		const url = new URL(window.location.href);
 		url.searchParams.set('cx', cx);
 		url.searchParams.set('cy', cy);
 		url.searchParams.set('z', z);
-		replaceState(url, page.state);
+		try {
+			replaceState(url, page.state);
+		} catch {
+			// Router not ready yet — the next view change retries.
+		}
 	});
 });
 </script>