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/es-to-primitive/helpers/isPrimitive.js

@@ -0,0 +1,9 @@
+'use strict';
+
+/** @import { primitive } from '../' */
+/** @import { primitiveES5 } from '../es5' */
+
+/** @type {<T extends primitive | primitiveES5>(value: unknown) => value is T} */
+module.exports = function isPrimitive(value) {
+	return value === null || (typeof value !== 'function' && typeof value !== 'object');
+};