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/2025/IsStringWellFormedUnicode.js

@@ -0,0 +1,23 @@
+'use strict';
+
+var CodePointAt = require('./CodePointAt');
+
+var $TypeError = require('es-errors/type');
+
+// https://262.ecma-international.org/14.0/#sec-isstringwellformedunicode
+
+module.exports = function IsStringWellFormedUnicode(string) {
+	if (typeof string !== 'string') {
+		throw new $TypeError('Assertion failed: `string` must be a String');
+	}
+	var len = string.length; // step 1
+	var k = 0; // step 2
+	while (k < len) { // step 3
+		var cp = CodePointAt(string, k); // step 3.a
+		if (cp['[[IsUnpairedSurrogate]]']) {
+			return false; // step 3.b
+		}
+		k += cp['[[CodeUnitCount]]']; // step 3.c
+	}
+	return true; // step 4
+};