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/supports-preserve-symlinks-flag/index.js

@@ -0,0 +1,9 @@
+'use strict';
+
+module.exports = (
+// node 12+
+	process.allowedNodeEnvironmentFlags && process.allowedNodeEnvironmentFlags.has('--preserve-symlinks')
+) || (
+// node v6.2 - v11
+	String(module.constructor._findPath).indexOf('preserveSymlinks') >= 0 // eslint-disable-line no-underscore-dangle
+);