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/helpers/every.js

@@ -0,0 +1,10 @@
+'use strict';
+
+module.exports = function every(array, predicate) {
+	for (var i = 0; i < array.length; i += 1) {
+		if (!predicate(array[i], i, array)) {
+			return false;
+		}
+	}
+	return true;
+};