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/2018/SameValue.js

@@ -0,0 +1,13 @@
+'use strict';
+
+var $isNaN = require('math-intrinsics/isNaN');
+
+// http://262.ecma-international.org/5.1/#sec-9.12
+
+module.exports = function SameValue(x, y) {
+	if (x === y) { // 0 === -0, but they are not identical.
+		if (x === 0) { return 1 / x === 1 / y; }
+		return true;
+	}
+	return $isNaN(x) && $isNaN(y);
+};