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/resolve/index.js

@@ -0,0 +1,6 @@
+var async = require('./lib/async');
+async.core = require('./lib/core');
+async.isCore = require('./lib/is-core');
+async.sync = require('./lib/sync');
+
+module.exports = async;