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-abstract/helpers/isStringOrHole.js

@@ -0,0 +1,9 @@
+'use strict';
+
+// TODO: semver-major: remove
+
+var canDistinguishSparseFromUndefined = 0 in [undefined]; // IE 6 - 8 have a bug where this returns false
+
+module.exports = function isStringOrHole(item, index, arr) {
+	return typeof item === 'string' || (canDistinguishSparseFromUndefined ? !(index in arr) : typeof item === 'undefined');
+};