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/async-function/index.d.ts

@@ -0,0 +1,13 @@
+declare namespace getAsyncFunction {
+    type AsyncFunction<T = unknown> = (...args: any[]) => Promise<T>;
+
+    interface AsyncFunctionConstructor extends FunctionConstructor {
+        new <T>(...args: string[]): AsyncFunction<T>;
+        <T>(...args: string[]): AsyncFunction<T>;
+        readonly prototype: AsyncFunction;
+    }
+}
+
+declare function getAsyncFunction(): getAsyncFunction.AsyncFunctionConstructor | false;
+
+export = getAsyncFunction;