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/2019/StrictEqualityComparison.js

@@ -0,0 +1,15 @@
+'use strict';
+
+var Type = require('./Type');
+
+// https://262.ecma-international.org/5.1/#sec-11.9.6
+
+module.exports = function StrictEqualityComparison(x, y) {
+	if (Type(x) !== Type(y)) {
+		return false;
+	}
+	if (typeof x === 'undefined' || x === null) {
+		return true;
+	}
+	return x === y; // shortcut for steps 4-7
+};