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/CharacterComplement.js

@@ -0,0 +1,34 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var AllCharacters = require('./AllCharacters');
+
+var CharSet = require('../helpers/CharSet').CharSet;
+var isRegExpRecord = require('../helpers/records/regexp-record');
+
+// https://262.ecma-international.org/15.0/#sec-charactercomplement
+
+module.exports = function CharacterComplement(rer, S) {
+	if (!isRegExpRecord(rer)) {
+		throw new $TypeError('Assertion failed: `rer` must be a RegExp Record');
+	}
+
+	if (!(S instanceof CharSet)) {
+		throw new $TypeError('Assertion failed: S must be a CharSet');
+	}
+
+	var A = AllCharacters(rer); // step 1
+
+	// 2. Return the CharSet containing the CharSetElements of A which are not also CharSetElements of S.
+	return new CharSet(
+		function (x) { return !S.test(x) && A.test(x); },
+		function (emit) {
+			A.yield(function (x) {
+				if (!S.test(x)) {
+					emit(x);
+				}
+			});
+		}
+	);
+};