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/is-module/index.js

@@ -0,0 +1,11 @@
+
+// no idea what these regular expressions do,
+// but i extracted it from https://github.com/yahoo/js-module-formats/blob/master/index.js#L18
+var ES6ImportExportRegExp = /(?:^\s*|[}{\(\);,\n]\s*)(import\s+['"]|(import|module)\s+[^"'\(\)\n;]+\s+from\s+['"]|export\s+(\*|\{|default|function|var|const|let|[_$a-zA-Z\xA0-\uFFFF][_$a-zA-Z0-9\xA0-\uFFFF]*))/;
+
+var ES6AliasRegExp = /(?:^\s*|[}{\(\);,\n]\s*)(export\s*\*\s*from\s*(?:'([^']+)'|"([^"]+)"))/;
+
+module.exports = function (sauce) {
+  return ES6ImportExportRegExp.test(sauce)
+    || ES6AliasRegExp.test(sauce);
+};