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/es-abstract-get/GetMethod.d.ts

@@ -0,0 +1,13 @@
+import { PropertyKey } from './isPropertyKey';
+
+type GoodMethodKey<O, K extends PropertyKey> =
+    K extends keyof O
+        ? (O[K] extends Function | null | undefined ? K : never)
+        : K;
+
+declare function GetMethod<O extends {}, K extends PropertyKey>(
+    V: O,
+    P: K & GoodMethodKey<O, K>,
+): Function | undefined;
+
+export = GetMethod;