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/internal/createTester.js

@@ -0,0 +1,40 @@
+'use strict';
+
+Object.defineProperty(exports, "__esModule", {
+    value: true
+});
+exports.default = _createTester;
+
+var _breakLoop = require('./breakLoop.js');
+
+var _breakLoop2 = _interopRequireDefault(_breakLoop);
+
+var _wrapAsync = require('./wrapAsync.js');
+
+var _wrapAsync2 = _interopRequireDefault(_wrapAsync);
+
+function _interopRequireDefault(obj) { return obj && obj.__esModule ? obj : { default: obj }; }
+
+function _createTester(check, getResult) {
+    return (eachfn, arr, _iteratee, cb) => {
+        var testPassed = false;
+        var testResult;
+        const iteratee = (0, _wrapAsync2.default)(_iteratee);
+        eachfn(arr, (value, _, callback) => {
+            iteratee(value, (err, result) => {
+                if (err || err === false) return callback(err);
+
+                if (check(result) && !testResult) {
+                    testPassed = true;
+                    testResult = getResult(true, value);
+                    return callback(null, _breakLoop2.default);
+                }
+                callback();
+            });
+        }, err => {
+            if (err) return cb(err);
+            cb(null, testPassed ? testResult : getResult(false));
+        });
+    };
+}
+module.exports = exports.default;