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/2021/ToNumeric.js

@@ -0,0 +1,20 @@
+'use strict';
+
+var GetIntrinsic = require('get-intrinsic');
+
+var $Number = GetIntrinsic('%Number%');
+
+var isPrimitive = require('../helpers/isPrimitive');
+
+var ToPrimitive = require('./ToPrimitive');
+var ToNumber = require('./ToNumber');
+
+// https://262.ecma-international.org/11.0/#sec-tonumeric
+
+module.exports = function ToNumeric(argument) {
+	var primValue = isPrimitive(argument) ? argument : ToPrimitive(argument, $Number);
+	if (typeof primValue === 'bigint') {
+		return primValue;
+	}
+	return ToNumber(primValue);
+};