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-range-requests/createPartialResponse.d.ts

@@ -0,0 +1,21 @@
+import './_version.js';
+/**
+ * Given a `Request` and `Response` objects as input, this will return a
+ * promise for a new `Response`.
+ *
+ * If the original `Response` already contains partial content (i.e. it has
+ * a status of 206), then this assumes it already fulfills the `Range:`
+ * requirements, and will return it as-is.
+ *
+ * @param {Request} request A request, which should contain a Range:
+ * header.
+ * @param {Response} originalResponse A response.
+ * @return {Promise<Response>} Either a `206 Partial Content` response, with
+ * the response body set to the slice of content specified by the request's
+ * `Range:` header, or a `416 Range Not Satisfiable` response if the
+ * conditions of the `Range:` header can't be met.
+ *
+ * @memberof workbox-range-requests
+ */
+declare function createPartialResponse(request: Request, originalResponse: Response): Promise<Response>;
+export { createPartialResponse };