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/isexe/windows.js

@@ -0,0 +1,42 @@
+module.exports = isexe
+isexe.sync = sync
+
+var fs = require('fs')
+
+function checkPathExt (path, options) {
+  var pathext = options.pathExt !== undefined ?
+    options.pathExt : process.env.PATHEXT
+
+  if (!pathext) {
+    return true
+  }
+
+  pathext = pathext.split(';')
+  if (pathext.indexOf('') !== -1) {
+    return true
+  }
+  for (var i = 0; i < pathext.length; i++) {
+    var p = pathext[i].toLowerCase()
+    if (p && path.substr(-p.length).toLowerCase() === p) {
+      return true
+    }
+  }
+  return false
+}
+
+function checkStat (stat, path, options) {
+  if (!stat.isSymbolicLink() && !stat.isFile()) {
+    return false
+  }
+  return checkPathExt(path, options)
+}
+
+function isexe (path, options, cb) {
+  fs.stat(path, function (er, stat) {
+    cb(er, er ? false : checkStat(stat, path, options))
+  })
+}
+
+function sync (path, options) {
+  return checkStat(fs.statSync(path), path, options)
+}