feat(pwa): Sync-Status-Store mit localStorage-Persistierung

Spiegelt SW-Messages (sync-start/progress/done/error) in einen
Svelte-State. lastSynced wird in localStorage persistiert, damit
der User nach einem Reload sieht, wann zuletzt synchronisiert
wurde. Wird vom SyncIndicator und der Admin-App-Tab konsumiert.

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

tests/unit/sync-status-store.test.ts

@@ -0,0 +1,30 @@
+import { describe, it, expect, beforeEach } from 'vitest';
+
+describe('sync-status store', () => {
+  beforeEach(async () => {
+    const mod = await import('../../src/lib/client/sync-status.svelte');
+    mod.syncStatus.state = { kind: 'idle' };
+    mod.syncStatus.lastSynced = null;
+  });
+
+  it('processes progress messages', async () => {
+    const { syncStatus } = await import('../../src/lib/client/sync-status.svelte');
+    syncStatus.handle({ type: 'sync-progress', current: 3, total: 10 });
+    expect(syncStatus.state).toEqual({ kind: 'syncing', current: 3, total: 10 });
+  });
+
+  it('transitions to idle on sync-done and records timestamp', async () => {
+    const { syncStatus } = await import('../../src/lib/client/sync-status.svelte');
+    syncStatus.handle({ type: 'sync-start', total: 5 });
+    expect(syncStatus.state.kind).toBe('syncing');
+    syncStatus.handle({ type: 'sync-done', lastSynced: 1700000000000 });
+    expect(syncStatus.state).toEqual({ kind: 'idle' });
+    expect(syncStatus.lastSynced).toBe(1700000000000);
+  });
+
+  it('sets error state on sync-error', async () => {
+    const { syncStatus } = await import('../../src/lib/client/sync-status.svelte');
+    syncStatus.handle({ type: 'sync-error', message: 'Quota exceeded' });
+    expect(syncStatus.state).toEqual({ kind: 'error', message: 'Quota exceeded' });
+  });
+});