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/temp-dir/index.js

@@ -0,0 +1,13 @@
+'use strict';
+const fs = require('fs');
+const os = require('os');
+
+const tempDirectorySymbol = Symbol.for('__RESOLVED_TEMP_DIRECTORY__');
+
+if (!global[tempDirectorySymbol]) {
+	Object.defineProperty(global, tempDirectorySymbol, {
+		value: fs.realpathSync(os.tmpdir())
+	});
+}
+
+module.exports = global[tempDirectorySymbol];