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/2020/UnicodeEscape.js

@@ -0,0 +1,25 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var callBound = require('call-bound');
+
+var $charCodeAt = callBound('String.prototype.charCodeAt');
+var $numberToString = callBound('Number.prototype.toString');
+var $toLowerCase = callBound('String.prototype.toLowerCase');
+
+var StringPad = require('./StringPad');
+
+// https://262.ecma-international.org/11.0/#sec-unicodeescape
+
+module.exports = function UnicodeEscape(C) {
+	if (typeof C !== 'string' || C.length !== 1) {
+		throw new $TypeError('Assertion failed: `C` must be a single code unit');
+	}
+	var n = $charCodeAt(C, 0);
+	if (n > 0xFFFF) {
+		throw new $TypeError('`Assertion failed: numeric value of `C` must be <= 0xFFFF');
+	}
+
+	return '\\u' + StringPad($toLowerCase($numberToString(n, 16)), 4, '0', 'start');
+};