fix(nav): scroll-restore key auf nav.from.url, nicht location

Live-Test auf kochwas-dev offenbarte: bei Browser-Back hat der Browser
die History bereits gepoppt, bevor SvelteKit beforeNavigate feuert —
location.pathname war damit schon die Ziel-URL. recordScroll() schrieb
also den 0-Wert der Recipe-Page in den Slot der Home-Page und wischte
den eigentlich gemerkten Wert (z. B. 500) raus. Restore las dann 0,
fiel unter MIN_RESTORE_Y und tat nichts.

Fix: recordScroll(nav.from?.url) und restoreScroll(type, nav.to?.url).
Helper bekommen die URL explizit reingereicht — keine Abhängigkeit
mehr von location und damit kein Race mit der Browser-History.

Tests: zusätzliche Regression "does not overwrite a stored URL when
called with a different from-url" plus Skip-Pfade fuer null URLs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/client/scroll-restore.ts

@@ -5,9 +5,11 @@
 // wishlist, shopping-list) are still empty at that point, so the saved
 // scrollY can't be reached and the browser clamps to 0.
 //
-// We patch this by saving scrollY on beforeNavigate and re-applying it
-// after popstate as soon as the document is tall enough — using rAF
-// polling with a hard time budget so we never spin.
+// We patch this by saving scrollY on beforeNavigate (keyed by the URL
+// we're leaving — NOT location.pathname, which on popstate is already
+// the new URL by the time the callback fires) and re-applying it after
+// popstate as soon as the document is tall enough — rAF-polled with a
+// hard time budget so we never spin.
 
 const STORAGE_KEY = 'kochwas:scroll';
 const POLL_BUDGET_MS = 1500;
@@ -34,26 +36,26 @@ function writeMap(map: ScrollMap): void {
   }
 }
 
-function urlKey(): string {
-  if (typeof location === 'undefined') return '';
-  return location.pathname + location.search;
+function keyFor(url: URL): string {
+  return url.pathname + url.search;
 }
 
-export function recordScroll(): void {
+export function recordScroll(fromUrl: URL | null | undefined): void {
   if (typeof window === 'undefined') return;
-  const key = urlKey();
-  if (!key) return;
+  if (!fromUrl) return;
   const map = readMap();
-  map[key] = window.scrollY;
+  map[keyFor(fromUrl)] = window.scrollY;
   writeMap(map);
 }
 
-export function restoreScroll(navType: string | null | undefined): void {
+export function restoreScroll(
+  navType: string | null | undefined,
+  toUrl: URL | null | undefined
+): void {
   if (typeof window === 'undefined') return;
   if (navType !== 'popstate') return;
-  const key = urlKey();
-  if (!key) return;
-  const target = readMap()[key];
+  if (!toUrl) return;
+  const target = readMap()[keyFor(toUrl)];
   if (!target || target < MIN_RESTORE_Y) return;
 
   const start = performance.now();

src/routes/+layout.svelte

@@ -98,8 +98,8 @@
     }
   }
 
-  beforeNavigate(() => {
-    recordScroll();
+  beforeNavigate((nav) => {
+    recordScroll(nav.from?.url);
   });
 
   afterNavigate((nav) => {
@@ -112,7 +112,7 @@
     // wurde.
     void wishlistStore.refresh();
     void shoppingCartStore.refresh();
-    restoreScroll(nav.type);
+    restoreScroll(nav.type, nav.to?.url);
   });
 
   onMount(() => {