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/fs-extra/lib/json/output-json-sync.js

@@ -0,0 +1,12 @@
+'use strict'
+
+const { stringify } = require('jsonfile/utils')
+const { outputFileSync } = require('../output')
+
+function outputJsonSync (file, data, options) {
+  const str = stringify(data, options)
+
+  outputFileSync(file, str, options)
+}
+
+module.exports = outputJsonSync