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/mkdirs/index.js

@@ -0,0 +1,14 @@
+'use strict'
+const u = require('universalify').fromPromise
+const { makeDir: _makeDir, makeDirSync } = require('./make-dir')
+const makeDir = u(_makeDir)
+
+module.exports = {
+  mkdirs: makeDir,
+  mkdirsSync: makeDirSync,
+  // alias
+  mkdirp: makeDir,
+  mkdirpSync: makeDirSync,
+  ensureDir: makeDir,
+  ensureDirSync: makeDirSync
+}