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/es-abstract/helpers/isNegativeZero.js

@@ -0,0 +1,6 @@
+'use strict';
+
+// TODO, semver-major: remove
+module.exports = function isNegativeZero(x) {
+	return x === 0 && 1 / x === 1 / -0;
+};