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/safe-push-apply/index.js

@@ -0,0 +1,15 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var isArray = require('isarray');
+
+/** @type {import('.')} */
+module.exports = function safePushApply(target, source) {
+	if (!isArray(target)) {
+		throw new $TypeError('target must be an array');
+	}
+	for (var i = 0; i < source.length; i++) {
+		target[target.length] = source[i]; // eslint-disable-line no-param-reassign
+	}
+};