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/index.js

@@ -0,0 +1,27 @@
+'use strict'
+
+module.exports = {
+  // Export promiseified graceful-fs:
+  ...require('./fs'),
+  // Export extra methods:
+  ...require('./copy-sync'),
+  ...require('./copy'),
+  ...require('./empty'),
+  ...require('./ensure'),
+  ...require('./json'),
+  ...require('./mkdirs'),
+  ...require('./move-sync'),
+  ...require('./move'),
+  ...require('./output'),
+  ...require('./path-exists'),
+  ...require('./remove')
+}
+
+// Export fs.promises as a getter property so that we don't trigger
+// ExperimentalWarning before fs.promises is actually accessed.
+const fs = require('fs')
+if (Object.getOwnPropertyDescriptor(fs, 'promises')) {
+  Object.defineProperty(module.exports, 'promises', {
+    get () { return fs.promises }
+  })
+}