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/is-document.all/eslint.config.mjs

@@ -0,0 +1,10 @@
+import ljharbConfig from '@ljharb/eslint-config/flat';
+
+export default [
+	...ljharbConfig,
+	{
+		rules: {
+			'no-extra-parens': 'off',
+		},
+	},
+];