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/createHandlerBoundToURL.d.ts

@@ -0,0 +1,21 @@
+import { RouteHandlerCallback } from 'workbox-core/types.js';
+import './_version.js';
+/**
+ * Helper function that calls
+ * {@link PrecacheController#createHandlerBoundToURL} on the default
+ * {@link PrecacheController} instance.
+ *
+ * If you are creating your own {@link PrecacheController}, then call the
+ * {@link PrecacheController#createHandlerBoundToURL} on that instance,
+ * instead of using this function.
+ *
+ * @param {string} url The precached URL which will be used to lookup the
+ * `Response`.
+ * @param {boolean} [fallbackToNetwork=true] Whether to attempt to get the
+ * response from the network if there's a precache miss.
+ * @return {workbox-routing~handlerCallback}
+ *
+ * @memberof workbox-precaching
+ */
+declare function createHandlerBoundToURL(url: string): RouteHandlerCallback;
+export { createHandlerBoundToURL };