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/2021/thisSymbolValue.js

@@ -0,0 +1,20 @@
+'use strict';
+
+var $SyntaxError = require('es-errors/syntax');
+var callBound = require('call-bound');
+
+var $SymbolValueOf = callBound('Symbol.prototype.valueOf', true);
+
+// https://262.ecma-international.org/9.0/#sec-thissymbolvalue
+
+module.exports = function thisSymbolValue(value) {
+	if (typeof value === 'symbol') {
+		return value;
+	}
+
+	if (!$SymbolValueOf) {
+		throw new $SyntaxError('Symbols are not supported; thisSymbolValue requires that `value` be a Symbol or a Symbol object');
+	}
+
+	return $SymbolValueOf(value);
+};