feat(pwa): Online-Status-Store

Reaktiver Store basierend auf navigator.onLine und den window-
Events online/offline. Kein aktives Heuristik-Probing — für
unseren Offline-PWA-Use-Case reicht der Browser-Status. Wird von
SyncIndicator und require-online-Helper konsumiert.

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

tests/unit/network-store.test.ts

@@ -0,0 +1,23 @@
+// @vitest-environment jsdom
+import { describe, it, expect, beforeEach } from 'vitest';
+
+describe('network store', () => {
+  beforeEach(() => {
+    // Reset module state for each test
+    Object.defineProperty(navigator, 'onLine', { value: true, configurable: true });
+  });
+
+  it('reflects initial navigator.onLine and reacts to events', async () => {
+    const { network } = await import('../../src/lib/client/network.svelte');
+    network.init();
+    expect(network.online).toBe(true);
+
+    Object.defineProperty(navigator, 'onLine', { value: false, configurable: true });
+    window.dispatchEvent(new Event('offline'));
+    expect(network.online).toBe(false);
+
+    Object.defineProperty(navigator, 'onLine', { value: true, configurable: true });
+    window.dispatchEvent(new Event('online'));
+    expect(network.online).toBe(true);
+  });
+});