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/@apideck/better-ajv-errors/dist/index.js

@@ -0,0 +1,8 @@
+
+'use strict'
+
+if (process.env.NODE_ENV === 'production') {
+  module.exports = require('./better-ajv-errors.cjs.production.min.js')
+} else {
+  module.exports = require('./better-ajv-errors.cjs.development.js')
+}