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-routing/setCatchHandler.d.ts

@@ -0,0 +1,13 @@
+import { RouteHandler } from 'workbox-core/types.js';
+import './_version.js';
+/**
+ * If a Route throws an error while handling a request, this `handler`
+ * will be called and given a chance to provide a response.
+ *
+ * @param {workbox-routing~handlerCallback} handler A callback
+ * function that returns a Promise resulting in a Response.
+ *
+ * @memberof workbox-routing
+ */
+declare function setCatchHandler(handler: RouteHandler): void;
+export { setCatchHandler };