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/@babel/helper-define-polyfill-provider/lib/define-provider.js

@@ -0,0 +1,9 @@
+"use strict";
+
+exports.__esModule = true;
+exports.defineProvider = defineProvider;
+function defineProvider(factory) {
+  // This will allow us to do some things
+
+  return factory;
+}