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/2021/DeletePropertyOrThrow.js

@@ -0,0 +1,25 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+var isObject = require('es-object-atoms/isObject');
+
+var isPropertyKey = require('../helpers/isPropertyKey');
+
+// https://262.ecma-international.org/6.0/#sec-deletepropertyorthrow
+
+module.exports = function DeletePropertyOrThrow(O, P) {
+	if (!isObject(O)) {
+		throw new $TypeError('Assertion failed: Type(O) is not Object');
+	}
+
+	if (!isPropertyKey(P)) {
+		throw new $TypeError('Assertion failed: P is not a Property Key');
+	}
+
+	// eslint-disable-next-line no-param-reassign
+	var success = delete O[P];
+	if (!success) {
+		throw new $TypeError('Attempt to delete property failed.');
+	}
+	return success;
+};