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/globalthis/implementation.browser.js

@@ -0,0 +1,11 @@
+/* eslint no-negated-condition: 0, no-new-func: 0 */
+
+'use strict';
+
+if (typeof self !== 'undefined') {
+	module.exports = self;
+} else if (typeof window !== 'undefined') {
+	module.exports = window;
+} else {
+	module.exports = Function('return this')();
+}