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-background-sync/index.d.ts

@@ -0,0 +1,25 @@
+import { BackgroundSyncPlugin } from './BackgroundSyncPlugin.js';
+import { Queue, QueueOptions } from './Queue.js';
+import { QueueStore } from './QueueStore.js';
+import { StorableRequest } from './StorableRequest.js';
+import './_version.js';
+interface SyncManager {
+    getTags(): Promise<string[]>;
+    register(tag: string): Promise<void>;
+}
+declare global {
+    interface ServiceWorkerRegistration {
+        readonly sync: SyncManager;
+    }
+    interface SyncEvent extends ExtendableEvent {
+        readonly lastChance: boolean;
+        readonly tag: string;
+    }
+    interface ServiceWorkerGlobalScopeEventMap {
+        sync: SyncEvent;
+    }
+}
+/**
+ * @module workbox-background-sync
+ */
+export { BackgroundSyncPlugin, Queue, QueueOptions, QueueStore, StorableRequest };