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/is-string/index.js

@@ -0,0 +1,31 @@
+'use strict';
+
+var callBound = require('call-bound');
+
+/** @type {(receiver: ThisParameterType<typeof String.prototype.valueOf>, ...args: Parameters<typeof String.prototype.valueOf>) => ReturnType<typeof String.prototype.valueOf>} */
+var $strValueOf = callBound('String.prototype.valueOf');
+
+/** @type {import('.')} */
+var tryStringObject = function tryStringObject(value) {
+	try {
+		$strValueOf(value);
+		return true;
+	} catch (e) {
+		return false;
+	}
+};
+/** @type {(receiver: ThisParameterType<typeof Object.prototype.toString>, ...args: Parameters<typeof Object.prototype.toString>) => ReturnType<typeof Object.prototype.toString>} */
+var $toString = callBound('Object.prototype.toString');
+var strClass = '[object String]';
+var hasToStringTag = require('has-tostringtag/shams')();
+
+/** @type {import('.')} */
+module.exports = function isString(value) {
+	if (typeof value === 'string') {
+		return true;
+	}
+	if (!value || typeof value !== 'object') {
+		return false;
+	}
+	return hasToStringTag ? tryStringObject(value) : $toString(value) === strClass;
+};