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-define-property/index.js

@@ -0,0 +1,14 @@
+'use strict';
+
+/** @type {import('.')} */
+var $defineProperty = Object.defineProperty || false;
+if ($defineProperty) {
+	try {
+		$defineProperty({}, 'a', { value: 1 });
+	} catch (e) {
+		// IE 8 has a broken defineProperty
+		$defineProperty = false;
+	}
+}
+
+module.exports = $defineProperty;