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>

tests/unit/scroll-restore.test.ts

@@ -0,0 +1,103 @@
+// @vitest-environment jsdom
+import { describe, it, expect, beforeEach, vi, afterEach } from 'vitest';
+
+import { recordScroll, restoreScroll } from '../../src/lib/client/scroll-restore';
+
+const STORAGE_KEY = 'kochwas:scroll';
+
+function setScrollY(y: number) {
+  Object.defineProperty(window, 'scrollY', { value: y, configurable: true });
+}
+
+function setDocHeight(h: number) {
+  Object.defineProperty(document.documentElement, 'scrollHeight', {
+    value: h,
+    configurable: true
+  });
+}
+
+function setViewportHeight(h: number) {
+  Object.defineProperty(window, 'innerHeight', { value: h, configurable: true });
+}
+
+describe('scroll-restore', () => {
+  let scrollToSpy: ReturnType<typeof vi.spyOn>;
+
+  beforeEach(() => {
+    sessionStorage.clear();
+    setScrollY(0);
+    setViewportHeight(800);
+    setDocHeight(800);
+    scrollToSpy = vi
+      .spyOn(window, 'scrollTo')
+      .mockImplementation(() => undefined as unknown as void);
+    window.history.replaceState({}, '', '/wishlist');
+  });
+
+  afterEach(() => {
+    scrollToSpy.mockRestore();
+    vi.useRealTimers();
+  });
+
+  it('records scrollY keyed by pathname+search', () => {
+    setScrollY(1200);
+    recordScroll();
+    const map = JSON.parse(sessionStorage.getItem(STORAGE_KEY) ?? '{}');
+    expect(map['/wishlist']).toBe(1200);
+  });
+
+  it('keeps separate entries per URL', () => {
+    setScrollY(500);
+    recordScroll();
+    window.history.replaceState({}, '', '/?q=hi');
+    setScrollY(900);
+    recordScroll();
+    const map = JSON.parse(sessionStorage.getItem(STORAGE_KEY) ?? '{}');
+    expect(map['/wishlist']).toBe(500);
+    expect(map['/?q=hi']).toBe(900);
+  });
+
+  it('skips restore for non-popstate navigation', () => {
+    sessionStorage.setItem(STORAGE_KEY, JSON.stringify({ '/wishlist': 1000 }));
+    setDocHeight(5000);
+    restoreScroll('link');
+    expect(scrollToSpy).not.toHaveBeenCalled();
+  });
+
+  it('skips restore when no entry stored', () => {
+    setDocHeight(5000);
+    restoreScroll('popstate');
+    expect(scrollToSpy).not.toHaveBeenCalled();
+  });
+
+  it('skips restore for trivial scrollY (noise)', () => {
+    sessionStorage.setItem(STORAGE_KEY, JSON.stringify({ '/wishlist': 10 }));
+    setDocHeight(5000);
+    restoreScroll('popstate');
+    expect(scrollToSpy).not.toHaveBeenCalled();
+  });
+
+  it('scrolls immediately when document is already tall enough', async () => {
+    sessionStorage.setItem(STORAGE_KEY, JSON.stringify({ '/wishlist': 1000 }));
+    setDocHeight(5000);
+    restoreScroll('popstate');
+    // rAF resolves on next microtask in jsdom
+    await new Promise((r) => requestAnimationFrame(() => r(null)));
+    expect(scrollToSpy).toHaveBeenCalledWith({ top: 1000, left: 0, behavior: 'instant' });
+  });
+
+  it('waits via rAF until document grows tall enough', async () => {
+    sessionStorage.setItem(STORAGE_KEY, JSON.stringify({ '/wishlist': 1500 }));
+    // Initially short — would clamp.
+    setDocHeight(900);
+    restoreScroll('popstate');
+    await new Promise((r) => requestAnimationFrame(() => r(null)));
+    expect(scrollToSpy).not.toHaveBeenCalled();
+
+    // Simulate async data loading and document growing.
+    setDocHeight(3000);
+    // Allow rAF to fire again.
+    await new Promise((r) => requestAnimationFrame(() => r(null)));
+    expect(scrollToSpy).toHaveBeenCalledWith({ top: 1500, left: 0, behavior: 'instant' });
+  });
+});