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/padTimeComponent.js

@@ -0,0 +1,9 @@
+'use strict';
+
+var callBound = require('call-bound');
+
+var $strSlice = callBound('String.prototype.slice');
+
+module.exports = function padTimeComponent(c, count) {
+	return $strSlice('00' + c, -(count || 2));
+};