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-broadcast-update/utils/constants.d.ts

@@ -0,0 +1,5 @@
+import '../_version.js';
+export declare const CACHE_UPDATED_MESSAGE_TYPE = "CACHE_UPDATED";
+export declare const CACHE_UPDATED_MESSAGE_META = "workbox-broadcast-update";
+export declare const NOTIFY_ALL_CLIENTS = true;
+export declare const DEFAULT_HEADERS_TO_CHECK: string[];

frontend/node_modules/workbox-broadcast-update/utils/constants.js

@@ -0,0 +1,16 @@
+/*
+  Copyright 2018 Google LLC
+
+  Use of this source code is governed by an MIT-style
+  license that can be found in the LICENSE file or at
+  https://opensource.org/licenses/MIT.
+*/
+import '../_version.js';
+export const CACHE_UPDATED_MESSAGE_TYPE = 'CACHE_UPDATED';
+export const CACHE_UPDATED_MESSAGE_META = 'workbox-broadcast-update';
+export const NOTIFY_ALL_CLIENTS = true;
+export const DEFAULT_HEADERS_TO_CHECK = [
+    'content-length',
+    'etag',
+    'last-modified',
+];

frontend/node_modules/workbox-broadcast-update/utils/constants.mjs

@@ -0,0 +1 @@
+export * from './constants.js';