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-async-function/index.d.ts

@@ -0,0 +1,9 @@
+import type { AsyncFunction } from 'async-function';
+
+declare namespace isAsyncFunction {
+    export type { AsyncFunction };
+}
+
+declare function isAsyncFunction(fn: unknown): fn is AsyncFunction;
+
+export = isAsyncFunction;