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.d.ts

@@ -0,0 +1,18 @@
+/**
+Get the real path of the system temp directory.
+
+@example
+```
+import * as os from 'os';
+import tempDirectory = require('temp-dir');
+
+console.log(tempDirectory);
+//=> '/private/var/folders/3x/jf5977fn79jbglr7rk0tq4d00000gn/T'
+
+console.log(os.tmpdir());
+//=> '/var/folders/3x/jf5977fn79jbglr7rk0tq4d00000gn/T' // <= Symlink
+```
+*/
+declare const tempDirectory: string;
+
+export = tempDirectory;