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/at-least-node/index.js

@@ -0,0 +1,5 @@
+module.exports = r => {
+  const n = process.versions.node.split('.').map(x => parseInt(x, 10))
+  r = r.split('.').map(x => parseInt(x, 10))
+  return n[0] > r[0] || (n[0] === r[0] && (n[1] > r[1] || (n[1] === r[1] && n[2] >= r[2])))
+}