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/promiseCallback.js

@@ -0,0 +1,23 @@
+'use strict';
+
+Object.defineProperty(exports, "__esModule", {
+    value: true
+});
+const PROMISE_SYMBOL = Symbol('promiseCallback');
+
+function promiseCallback() {
+    let resolve, reject;
+    function callback(err, ...args) {
+        if (err) return reject(err);
+        resolve(args.length > 1 ? args : args[0]);
+    }
+
+    callback[PROMISE_SYMBOL] = new Promise((res, rej) => {
+        resolve = res, reject = rej;
+    });
+
+    return callback;
+}
+
+exports.promiseCallback = promiseCallback;
+exports.PROMISE_SYMBOL = PROMISE_SYMBOL;