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/es5.js

@@ -0,0 +1,56 @@
+'use strict';
+
+var toStr = Object.prototype.toString;
+
+var isPrimitive = require('./helpers/isPrimitive');
+
+var isCallable = require('is-callable');
+var $TypeError = require('es-errors/type');
+
+/** @import { unknownES5 } from './es5' */
+
+// http://ecma-international.org/ecma-262/5.1/#sec-8.12.8
+var ES5internalSlots = /** @type {const} */ ({
+	'[[DefaultValue]]': /** @param {object} O */ function (O) {
+		var actualHint;
+		if (arguments.length > 1) {
+			actualHint = arguments[1];
+		} else {
+			actualHint = toStr.call(O) === '[object Date]' ? String : Number;
+		}
+
+		if (actualHint === String || actualHint === Number) {
+			var methods = actualHint === String
+				? /** @type {const} */ (['toString', 'valueOf'])
+				: /** @type {const} */ (['valueOf', 'toString']);
+			/** @type {unknownES5} */
+			var value;
+			for (var i = 0; i < methods.length; ++i) {
+				var method = methods[i];
+				if (isCallable(O[method])) {
+
+					value = O[method]();
+					if (isPrimitive(value)) {
+						return value;
+					}
+				}
+			}
+			throw new $TypeError('No default value');
+		}
+		throw new $TypeError('invalid [[DefaultValue]] hint supplied');
+	}
+});
+
+/** @type {import('./es5')} */
+// http://ecma-international.org/ecma-262/5.1/#sec-9.1
+module.exports = function ToPrimitive(input) {
+	if (isPrimitive(input)) {
+		return input;
+	}
+	if (arguments.length > 1) {
+
+		return ES5internalSlots['[[DefaultValue]]'](input, arguments[1]);
+	}
+
+	return ES5internalSlots['[[DefaultValue]]'](input);
+};