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/isSamePropertyDescriptor.js

@@ -0,0 +1,20 @@
+'use strict';
+
+var every = require('./every');
+
+module.exports = function isSamePropertyDescriptor(ES, D1, D2) {
+	var fields = [
+		'[[Configurable]]',
+		'[[Enumerable]]',
+		'[[Get]]',
+		'[[Set]]',
+		'[[Value]]',
+		'[[Writable]]'
+	];
+	return every(fields, function (field) {
+		if ((field in D1) !== (field in D2)) {
+			return false;
+		}
+		return ES.SameValue(D1[field], D2[field]);
+	});
+};