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/2015/ToDateString.js

@@ -0,0 +1,21 @@
+'use strict';
+
+var GetIntrinsic = require('get-intrinsic');
+
+var $TypeError = require('es-errors/type');
+var $Date = GetIntrinsic('%Date%');
+var $String = GetIntrinsic('%String%');
+
+var $isNaN = require('math-intrinsics/isNaN');
+
+// https://262.ecma-international.org/6.0/#sec-todatestring
+
+module.exports = function ToDateString(tv) {
+	if (typeof tv !== 'number') {
+		throw new $TypeError('Assertion failed: `tv` must be a Number');
+	}
+	if ($isNaN(tv)) {
+		return 'Invalid Date';
+	}
+	return $String(new $Date(tv));
+};