fix(nav): Scroll-Position bei Browser-Back robust wiederherstellen

Pages, die ihre Daten in onMount per fetch laden (Home, Wunschliste,
Einkaufsliste), waren bei popstate-Navigation kaputt: SvelteKit ruft
scrollTo() synchron nach Mount, aber die Listen sind dann noch leer
und das Dokument zu kurz — der Browser clamped auf 0.

Neuer Helper src/lib/client/scroll-restore.ts merkt scrollY pro URL in
sessionStorage (beforeNavigate) und stellt sie bei popstate per rAF-
Polling wieder her, sobald document.scrollHeight gross genug ist
(Hard-Budget 1.5s, danach best-effort scrollTo).

Layout ruft die zwei Helper im beforeNavigate / afterNavigate. Pages
mit SSR-Daten (z. B. /recipes) bleiben unbeeinflusst — dort matcht
unser Wert SvelteKits eigenen scrollTo bereits beim ersten Frame.

Tests: 7 neue Unit-Tests in tests/unit/scroll-restore.test.ts decken
Recording, Pro-URL-Trennung, Skip fuer Forward-Nav, sofortiges und
verzoegertes Restore ab.

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

src/lib/client/scroll-restore.ts

@@ -0,0 +1,76 @@
+// Persistent scroll restoration across client navigations.
+//
+// SvelteKit only restores scroll synchronously after the new page mounts.
+// Pages whose content is fetched in onMount/afterNavigate (e.g. home,
+// 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.
+
+const STORAGE_KEY = 'kochwas:scroll';
+const POLL_BUDGET_MS = 1500;
+const MIN_RESTORE_Y = 40; // ignore noise: don't override a default top scroll
+
+type ScrollMap = Record<string, number>;
+
+function readMap(): ScrollMap {
+  if (typeof sessionStorage === 'undefined') return {};
+  try {
+    const raw = sessionStorage.getItem(STORAGE_KEY);
+    return raw ? (JSON.parse(raw) as ScrollMap) : {};
+  } catch {
+    return {};
+  }
+}
+
+function writeMap(map: ScrollMap): void {
+  if (typeof sessionStorage === 'undefined') return;
+  try {
+    sessionStorage.setItem(STORAGE_KEY, JSON.stringify(map));
+  } catch {
+    // quota exceeded — silently drop, scroll memory is best-effort
+  }
+}
+
+function urlKey(): string {
+  if (typeof location === 'undefined') return '';
+  return location.pathname + location.search;
+}
+
+export function recordScroll(): void {
+  if (typeof window === 'undefined') return;
+  const key = urlKey();
+  if (!key) return;
+  const map = readMap();
+  map[key] = window.scrollY;
+  writeMap(map);
+}
+
+export function restoreScroll(navType: string | null | undefined): void {
+  if (typeof window === 'undefined') return;
+  if (navType !== 'popstate') return;
+  const key = urlKey();
+  if (!key) return;
+  const target = readMap()[key];
+  if (!target || target < MIN_RESTORE_Y) return;
+
+  const start = performance.now();
+  const step = () => {
+    const docHeight = document.documentElement.scrollHeight;
+    const reachable = Math.max(0, docHeight - window.innerHeight);
+    if (reachable >= target - 4) {
+      window.scrollTo({ top: target, left: 0, behavior: 'instant' });
+      return;
+    }
+    if (performance.now() - start >= POLL_BUDGET_MS) {
+      // Best effort — content never grew tall enough; clamp will land us
+      // at the bottom of what's available.
+      window.scrollTo({ top: target, left: 0, behavior: 'instant' });
+      return;
+    }
+    requestAnimationFrame(step);
+  };
+  requestAnimationFrame(step);
+}

src/routes/+layout.svelte

@@ -1,7 +1,7 @@
 <script lang="ts">
   import { onMount } from 'svelte';
   import { page } from '$app/stores';
-  import { goto, afterNavigate } from '$app/navigation';
+  import { goto, afterNavigate, beforeNavigate } from '$app/navigation';
   import {
     Settings,
     CookingPot,
@@ -28,6 +28,7 @@
   import { installPrompt } from '$lib/client/install-prompt.svelte';
   import { registerServiceWorker } from '$lib/client/sw-register';
   import { SearchStore } from '$lib/client/search.svelte';
+  import { recordScroll, restoreScroll } from '$lib/client/scroll-restore';
 
   let { data, children } = $props();
 
@@ -97,7 +98,11 @@
     }
   }
 
-  afterNavigate(() => {
+  beforeNavigate(() => {
+    recordScroll();
+  });
+
+  afterNavigate((nav) => {
     navStore.reset();
     navOpen = false;
     menuOpen = false;
@@ -107,6 +112,7 @@
     // wurde.
     void wishlistStore.refresh();
     void shoppingCartStore.refresh();
+    restoreScroll(nav.type);
   });
 
   onMount(() => {