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/core-js-compat/index.js

@@ -0,0 +1,14 @@
+'use strict';
+const compat = require('./compat');
+const data = require('./data');
+const entries = require('./entries');
+const getModulesListForTargetVersion = require('./get-modules-list-for-target-version');
+const modules = require('./modules');
+
+module.exports = Object.assign(compat, {
+  compat,
+  data,
+  entries,
+  getModulesListForTargetVersion,
+  modules,
+});