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/timesLimit.js

@@ -0,0 +1,43 @@
+'use strict';
+
+Object.defineProperty(exports, "__esModule", {
+    value: true
+});
+exports.default = timesLimit;
+
+var _mapLimit = require('./mapLimit.js');
+
+var _mapLimit2 = _interopRequireDefault(_mapLimit);
+
+var _range = require('./internal/range.js');
+
+var _range2 = _interopRequireDefault(_range);
+
+var _wrapAsync = require('./internal/wrapAsync.js');
+
+var _wrapAsync2 = _interopRequireDefault(_wrapAsync);
+
+function _interopRequireDefault(obj) { return obj && obj.__esModule ? obj : { default: obj }; }
+
+/**
+ * The same as [times]{@link module:ControlFlow.times} but runs a maximum of `limit` async operations at a
+ * time.
+ *
+ * @name timesLimit
+ * @static
+ * @memberOf module:ControlFlow
+ * @method
+ * @see [async.times]{@link module:ControlFlow.times}
+ * @category Control Flow
+ * @param {number} count - The number of times to run the function.
+ * @param {number} limit - The maximum number of async operations at a time.
+ * @param {AsyncFunction} iteratee - The async function to call `n` times.
+ * Invoked with the iteration index and a callback: (n, next).
+ * @param {Function} callback - see [async.map]{@link module:Collections.map}.
+ * @returns {Promise} a promise, if no callback is provided
+ */
+function timesLimit(count, limit, iteratee, callback) {
+    var _iteratee = (0, _wrapAsync2.default)(iteratee);
+    return (0, _mapLimit2.default)((0, _range2.default)(count), limit, _iteratee, callback);
+}
+module.exports = exports.default;