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-function/index.js

@@ -0,0 +1,8 @@
+'use strict';
+
+// eslint-disable-next-line no-extra-parens, no-empty-function
+const cached = /** @type {import('.').AsyncFunctionConstructor} */ (async function () {}.constructor);
+
+/** @type {import('.')} */
+module.exports = () => cached;
+