feat(PBI-74): hidden-tab + reconnect resync (Story 5)

Per ontwerp samen in één commit zodat geen vangnet wegvalt zonder vervanging.

- T-861: useBacklogRealtime sluit niet meer op visibilitychange hidden;
  EventSource blijft open zolang browser/netwerk dit toelaten. Reconnect bij
  netwerkfout blijft via backoff. visibilitychange fungeert nog wel als
  re-connect-trigger als de stream tussentijds is gesloten (b.v. 240s
  hard-close server-side).
- T-862: 'ready'-event-handler telt connect-cycles. De eerste 'ready' is de
  initial connect (geen resync). Bij latere 'ready' (post-reconnect) wordt
  resyncActiveScopes('reconnect') aangeroepen om gemiste events op te halen.
- T-863: nieuwe lib/realtime/use-workspace-resync.ts — luistert op
  document.visibilitychange (hidden→visible) en window.online; dispatcht
  resyncActiveScopes('visible') resp. 'reconnect'. Mounted in
  BacklogHydrationWrapper na useBacklogRealtime.
- T-864: 4 nieuwe vitest-cases voor useWorkspaceResync (jsdom): visible→
  visible event, online event, hidden negeren, cleanup-bij-unmount.

Daarnaast lint-cleanup: ongebruikte 'order'-variabelen in pbi-list en
story-panel weggehaald.

Verify: lint+typecheck clean, 646/646 tests groen.

Refs: PBI-74, ST-1322, T-861..T-864

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

__tests__/realtime/use-workspace-resync.test.tsx

@@ -0,0 +1,69 @@
+// @vitest-environment jsdom
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import { renderHook } from '@testing-library/react'
+
+import { useProductWorkspaceStore } from '@/stores/product-workspace/store'
+import { useWorkspaceResync } from '@/lib/realtime/use-workspace-resync'
+
+let resyncSpy: ReturnType<typeof vi.fn>
+
+beforeEach(() => {
+  resyncSpy = vi.fn().mockResolvedValue(undefined)
+  useProductWorkspaceStore.setState((s) => {
+    s.resyncActiveScopes = resyncSpy as unknown as typeof s.resyncActiveScopes
+  })
+  // visibilitychange handler leest document.visibilityState — default is 'visible'
+  Object.defineProperty(document, 'visibilityState', {
+    value: 'visible',
+    writable: true,
+    configurable: true,
+  })
+})
+
+afterEach(() => {
+  vi.restoreAllMocks()
+})
+
+describe('useWorkspaceResync', () => {
+  it('triggert resyncActiveScopes("visible") op visibilitychange hidden→visible', () => {
+    renderHook(() => useWorkspaceResync())
+
+    Object.defineProperty(document, 'visibilityState', {
+      value: 'visible',
+      writable: true,
+      configurable: true,
+    })
+    document.dispatchEvent(new Event('visibilitychange'))
+
+    expect(resyncSpy).toHaveBeenCalledWith('visible')
+  })
+
+  it('triggert resyncActiveScopes("reconnect") op online-event', () => {
+    renderHook(() => useWorkspaceResync())
+    window.dispatchEvent(new Event('online'))
+    expect(resyncSpy).toHaveBeenCalledWith('reconnect')
+  })
+
+  it('triggert geen resync bij visibilitychange naar hidden', () => {
+    renderHook(() => useWorkspaceResync())
+
+    Object.defineProperty(document, 'visibilityState', {
+      value: 'hidden',
+      writable: true,
+      configurable: true,
+    })
+    document.dispatchEvent(new Event('visibilitychange'))
+
+    expect(resyncSpy).not.toHaveBeenCalled()
+  })
+
+  it('cleanup verwijdert listeners bij unmount', () => {
+    const { unmount } = renderHook(() => useWorkspaceResync())
+    unmount()
+
+    window.dispatchEvent(new Event('online'))
+    document.dispatchEvent(new Event('visibilitychange'))
+
+    expect(resyncSpy).not.toHaveBeenCalled()
+  })
+})