fix(pwa): Kaltstart nach verpassten Updates stabilisieren

Service Worker übernimmt Updates zuverlässig (SKIP_WAITING, clientsClaim),
wartende Versionen werden beim Start angewendet und veraltete Chunks führen
nicht mehr zum Hängenbleiben.

Co-authored-by: Cursor <cursoragent@cursor.com>

client/src/services/pwaStartup.test.ts

@@ -0,0 +1,45 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+import {
+  markReloadAttempt,
+  recentlyAttemptedReload,
+  reconcileServiceWorkerOnStartup
+} from './pwaStartup.js'
+
+describe('pwaStartup reload guards', () => {
+  beforeEach(() => {
+    sessionStorage.clear()
+  })
+
+  it('blocks repeated reload attempts within the debounce window', () => {
+    expect(recentlyAttemptedReload(10_000)).toBe(false)
+    markReloadAttempt(10_000)
+    expect(recentlyAttemptedReload(12_000)).toBe(true)
+    expect(recentlyAttemptedReload(15_000)).toBe(false)
+  })
+})
+
+describe('reconcileServiceWorkerOnStartup', () => {
+  beforeEach(() => {
+    sessionStorage.clear()
+    vi.unstubAllEnvs()
+  })
+
+  it('returns false in dev mode', async () => {
+    vi.stubEnv('DEV', true)
+    await expect(reconcileServiceWorkerOnStartup()).resolves.toBe(false)
+  })
+
+  it('returns false when no waiting worker exists', async () => {
+    vi.stubEnv('DEV', false)
+    Object.defineProperty(navigator, 'serviceWorker', {
+      configurable: true,
+      value: {
+        controller: {},
+        getRegistration: vi.fn().mockResolvedValue({ waiting: null }),
+        addEventListener: vi.fn()
+      }
+    })
+
+    await expect(reconcileServiceWorkerOnStartup()).resolves.toBe(false)
+  })
+})