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/2023/IsIntegralNumber.js

@@ -0,0 +1,14 @@
+'use strict';
+
+var truncate = require('./truncate');
+
+var $isFinite = require('math-intrinsics/isFinite');
+
+// https://262.ecma-international.org/14.0/#sec-isintegralnumber
+
+module.exports = function IsIntegralNumber(argument) {
+	if (typeof argument !== 'number' || !$isFinite(argument)) {
+		return false;
+	}
+	return truncate(argument) === argument;
+};