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/runtime/helpers/regeneratorAsync.js

@@ -0,0 +1,8 @@
+var regeneratorAsyncGen = require("./regeneratorAsyncGen.js");
+function _regeneratorAsync(n, e, r, t, o) {
+  var a = regeneratorAsyncGen(n, e, r, t, o);
+  return a.next().then(function (n) {
+    return n.done ? n.value : a.next();
+  });
+}
+module.exports = _regeneratorAsync, module.exports.__esModule = true, module.exports["default"] = module.exports;