feat(pwa): Cache-Manifest-Diff-Funktion + Tests

Pure Funktion diffManifest(current, cached) → {toAdd, toRemove}.
Vom SW beim Update-Sync genutzt: neue Rezept-IDs nachladen,
gelöschte aus dem Cache räumen. 5 Tests decken add/remove/
beides/unchanged/empty-cache ab.

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

tests/unit/diff-manifest.test.ts

@@ -0,0 +1,34 @@
+import { describe, it, expect } from 'vitest';
+import { diffManifest } from '../../src/lib/sw/diff-manifest';
+
+describe('diffManifest', () => {
+  it('detects new IDs to add', () => {
+    const result = diffManifest([1, 2, 3, 4], [1, 2]);
+    expect(result.toAdd.sort()).toEqual([3, 4]);
+    expect(result.toRemove).toEqual([]);
+  });
+
+  it('detects removed IDs', () => {
+    const result = diffManifest([1, 2], [1, 2, 3, 4]);
+    expect(result.toAdd).toEqual([]);
+    expect(result.toRemove.sort()).toEqual([3, 4]);
+  });
+
+  it('detects both add and remove in one diff', () => {
+    const result = diffManifest([1, 3, 5], [1, 2, 3]);
+    expect(result.toAdd).toEqual([5]);
+    expect(result.toRemove).toEqual([2]);
+  });
+
+  it('returns empty arrays when identical', () => {
+    const result = diffManifest([1, 2, 3], [3, 2, 1]);
+    expect(result.toAdd).toEqual([]);
+    expect(result.toRemove).toEqual([]);
+  });
+
+  it('handles empty caches (first sync)', () => {
+    const result = diffManifest([1, 2], []);
+    expect(result.toAdd.sort()).toEqual([1, 2]);
+    expect(result.toRemove).toEqual([]);
+  });
+});