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/2018/SpeciesConstructor.js

@@ -0,0 +1,32 @@
+'use strict';
+
+var GetIntrinsic = require('get-intrinsic');
+
+var $species = GetIntrinsic('%Symbol.species%', true);
+var $TypeError = require('es-errors/type');
+var isObject = require('es-object-atoms/isObject');
+
+var IsConstructor = require('./IsConstructor');
+
+// https://262.ecma-international.org/6.0/#sec-speciesconstructor
+
+module.exports = function SpeciesConstructor(O, defaultConstructor) {
+	if (!isObject(O)) {
+		throw new $TypeError('Assertion failed: Type(O) is not Object');
+	}
+	var C = O.constructor;
+	if (typeof C === 'undefined') {
+		return defaultConstructor;
+	}
+	if (!isObject(C)) {
+		throw new $TypeError('O.constructor is not an Object');
+	}
+	var S = $species ? C[$species] : void 0;
+	if (S == null) {
+		return defaultConstructor;
+	}
+	if (IsConstructor(S)) {
+		return S;
+	}
+	throw new $TypeError('no constructor found');
+};