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/async/groupBySeries.js

@@ -0,0 +1,36 @@
+'use strict';
+
+Object.defineProperty(exports, "__esModule", {
+    value: true
+});
+exports.default = groupBySeries;
+
+var _groupByLimit = require('./groupByLimit.js');
+
+var _groupByLimit2 = _interopRequireDefault(_groupByLimit);
+
+function _interopRequireDefault(obj) { return obj && obj.__esModule ? obj : { default: obj }; }
+
+/**
+ * The same as [`groupBy`]{@link module:Collections.groupBy} but runs only a single async operation at a time.
+ *
+ * @name groupBySeries
+ * @static
+ * @memberOf module:Collections
+ * @method
+ * @see [async.groupBy]{@link module:Collections.groupBy}
+ * @category Collection
+ * @param {Array|Iterable|AsyncIterable|Object} coll - A collection to iterate over.
+ * @param {AsyncFunction} iteratee - An async function to apply to each item in
+ * `coll`.
+ * The iteratee should complete with a `key` to group the value under.
+ * Invoked with (value, callback).
+ * @param {Function} [callback] - A callback which is called when all `iteratee`
+ * functions have finished, or an error occurs. Result is an `Object` whose
+ * properties are arrays of values which returned the corresponding key.
+ * @returns {Promise} a promise, if no callback is passed
+ */
+function groupBySeries(coll, iteratee, callback) {
+    return (0, _groupByLimit2.default)(coll, 1, iteratee, callback);
+}
+module.exports = exports.default;