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/2024/TimeClip.js

@@ -0,0 +1,20 @@
+'use strict';
+
+var GetIntrinsic = require('get-intrinsic');
+
+var $Date = GetIntrinsic('%Date%');
+
+var $isFinite = require('math-intrinsics/isFinite');
+var abs = require('math-intrinsics/abs');
+
+var ToNumber = require('./ToNumber');
+
+// https://262.ecma-international.org/5.1/#sec-15.9.1.14
+
+module.exports = function TimeClip(time) {
+	if (!$isFinite(time) || abs(time) > 8.64e15) {
+		return NaN;
+	}
+	return +new $Date(ToNumber(time));
+};
+