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-navigation-preload/index.d.ts

@@ -0,0 +1,23 @@
+import { disable } from './disable.js';
+import { enable } from './enable.js';
+import { isSupported } from './isSupported.js';
+import './_version.js';
+interface NavigationPreloadState {
+    enabled?: boolean;
+    headerValue?: string;
+}
+interface NavigationPreloadManager {
+    disable(): Promise<void>;
+    enable(): Promise<void>;
+    getState(): Promise<NavigationPreloadState>;
+    setHeaderValue(value: string): Promise<void>;
+}
+declare global {
+    interface ServiceWorkerRegistration {
+        readonly navigationPreload: NavigationPreloadManager;
+    }
+}
+/**
+ * @module workbox-navigation-preload
+ */
+export { disable, enable, isSupported };