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-precaching/addRoute.d.ts

@@ -0,0 +1,19 @@
+import { PrecacheRouteOptions } from './_types.js';
+import './_version.js';
+/**
+ * Add a `fetch` listener to the service worker that will
+ * respond to
+ * [network requests]{@link https://developer.mozilla.org/en-US/docs/Web/API/Service_Worker_API/Using_Service_Workers#Custom_responses_to_requests}
+ * with precached assets.
+ *
+ * Requests for assets that aren't precached, the `FetchEvent` will not be
+ * responded to, allowing the event to fall through to other `fetch` event
+ * listeners.
+ *
+ * @param {Object} [options] See the {@link workbox-precaching.PrecacheRoute}
+ * options.
+ *
+ * @memberof workbox-precaching
+ */
+declare function addRoute(options?: PrecacheRouteOptions): void;
+export { addRoute };