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/crypto-random-string/index.js

@@ -0,0 +1,10 @@
+'use strict';
+const crypto = require('crypto');
+
+module.exports = length => {
+	if (!Number.isFinite(length)) {
+		throw new TypeError('Expected a finite number');
+	}
+
+	return crypto.randomBytes(Math.ceil(length / 2)).toString('hex').slice(0, length);
+};