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-object-atoms/RequireObjectCoercible.js

@@ -0,0 +1,11 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+/** @type {import('./RequireObjectCoercible')} */
+module.exports = function RequireObjectCoercible(value) {
+	if (value == null) {
+		throw new $TypeError((arguments.length > 0 && arguments[1]) || ('Cannot call method on ' + value));
+	}
+	return value;
+};