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/graceful-fs/clone.js

@@ -0,0 +1,23 @@
+'use strict'
+
+module.exports = clone
+
+var getPrototypeOf = Object.getPrototypeOf || function (obj) {
+  return obj.__proto__
+}
+
+function clone (obj) {
+  if (obj === null || typeof obj !== 'object')
+    return obj
+
+  if (obj instanceof Object)
+    var copy = { __proto__: getPrototypeOf(obj) }
+  else
+    var copy = Object.create(null)
+
+  Object.getOwnPropertyNames(obj).forEach(function (key) {
+    Object.defineProperty(copy, key, Object.getOwnPropertyDescriptor(obj, key))
+  })
+
+  return copy
+}