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/2022/YearFromTime.js

@@ -0,0 +1,16 @@
+'use strict';
+
+var GetIntrinsic = require('get-intrinsic');
+
+var $Date = GetIntrinsic('%Date%');
+
+var callBound = require('call-bound');
+
+var $getUTCFullYear = callBound('Date.prototype.getUTCFullYear');
+
+// https://262.ecma-international.org/5.1/#sec-15.9.1.3
+
+module.exports = function YearFromTime(t) {
+	// largest y such that this.TimeFromYear(y) <= t
+	return $getUTCFullYear(new $Date(t));
+};