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/ensure/index.js

@@ -0,0 +1,23 @@
+'use strict'
+
+const file = require('./file')
+const link = require('./link')
+const symlink = require('./symlink')
+
+module.exports = {
+  // file
+  createFile: file.createFile,
+  createFileSync: file.createFileSync,
+  ensureFile: file.createFile,
+  ensureFileSync: file.createFileSync,
+  // link
+  createLink: link.createLink,
+  createLinkSync: link.createLinkSync,
+  ensureLink: link.createLink,
+  ensureLinkSync: link.createLinkSync,
+  // symlink
+  createSymlink: symlink.createSymlink,
+  createSymlinkSync: symlink.createSymlinkSync,
+  ensureSymlink: symlink.createSymlink,
+  ensureSymlinkSync: symlink.createSymlinkSync
+}