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/lib/StorableRequest.d.ts

@@ -0,0 +1,53 @@
+import { MapLikeObject } from 'workbox-core/types.js';
+import '../_version.js';
+export interface RequestData extends MapLikeObject {
+    url: string;
+    headers: MapLikeObject;
+    body?: ArrayBuffer;
+}
+/**
+ * A class to make it easier to serialize and de-serialize requests so they
+ * can be stored in IndexedDB.
+ *
+ * Most developers will not need to access this class directly;
+ * it is exposed for advanced use cases.
+ */
+declare class StorableRequest {
+    private readonly _requestData;
+    /**
+     * Converts a Request object to a plain object that can be structured
+     * cloned or JSON-stringified.
+     *
+     * @param {Request} request
+     * @return {Promise<StorableRequest>}
+     */
+    static fromRequest(request: Request): Promise<StorableRequest>;
+    /**
+     * Accepts an object of request data that can be used to construct a
+     * `Request` but can also be stored in IndexedDB.
+     *
+     * @param {Object} requestData An object of request data that includes the
+     *     `url` plus any relevant properties of
+     *     [requestInit]{@link https://fetch.spec.whatwg.org/#requestinit}.
+     */
+    constructor(requestData: RequestData);
+    /**
+     * Returns a deep clone of the instances `_requestData` object.
+     *
+     * @return {Object}
+     */
+    toObject(): RequestData;
+    /**
+     * Converts this instance to a Request.
+     *
+     * @return {Request}
+     */
+    toRequest(): Request;
+    /**
+     * Creates and returns a deep clone of the instance.
+     *
+     * @return {StorableRequest}
+     */
+    clone(): StorableRequest;
+}
+export { StorableRequest };