Add sync error logging, extend error flash, fix unload saves

- Log workforce sync errors to backend stdout so they appear in docker logs
- Extend error flash from 5s to 15s so errors are readable
- Replace sendBeacon (POST-only) with keepalive fetch (PUT) on unload —
  sendBeacon was hitting 404s on all three unload saves

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/node_modules/workbox-window/utils/WorkboxEvent.d.ts

@@ -0,0 +1,39 @@
+import { WorkboxEventTarget } from './WorkboxEventTarget.js';
+import '../_version.js';
+/**
+ * A minimal `Event` subclass shim.
+ * This doesn't *actually* subclass `Event` because not all browsers support
+ * constructable `EventTarget`, and using a real `Event` will error.
+ * @private
+ */
+export declare class WorkboxEvent<K extends keyof WorkboxEventMap> {
+    type: K;
+    target?: WorkboxEventTarget;
+    sw?: ServiceWorker;
+    originalEvent?: Event;
+    isExternal?: boolean;
+    constructor(type: K, props: Omit<WorkboxEventMap[K], 'target' | 'type'>);
+}
+export interface WorkboxMessageEvent extends WorkboxEvent<'message'> {
+    data: any;
+    originalEvent: Event;
+    ports: readonly MessagePort[];
+}
+export interface WorkboxLifecycleEvent extends WorkboxEvent<keyof WorkboxLifecycleEventMap> {
+    isUpdate?: boolean;
+}
+export interface WorkboxLifecycleWaitingEvent extends WorkboxLifecycleEvent {
+    wasWaitingBeforeRegister?: boolean;
+}
+export interface WorkboxLifecycleEventMap {
+    installing: WorkboxLifecycleEvent;
+    installed: WorkboxLifecycleEvent;
+    waiting: WorkboxLifecycleWaitingEvent;
+    activating: WorkboxLifecycleEvent;
+    activated: WorkboxLifecycleEvent;
+    controlling: WorkboxLifecycleEvent;
+    redundant: WorkboxLifecycleEvent;
+}
+export interface WorkboxEventMap extends WorkboxLifecycleEventMap {
+    message: WorkboxMessageEvent;
+}