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/escalade/sync/index.d.mts

@@ -0,0 +1,9 @@
+export type Callback = (
+	directory: string,
+	files: string[],
+) => string | false | void;
+
+export default function (
+	directory: string,
+	callback: Callback,
+): string | void;