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/define-data-property/index.d.ts

@@ -0,0 +1,12 @@
+
+declare function defineDataProperty(
+    obj: Record<PropertyKey, unknown>,
+    property: keyof typeof obj,
+    value: typeof obj[typeof property],
+    nonEnumerable?: boolean | null,
+    nonWritable?: boolean | null,
+    nonConfigurable?: boolean | null,
+    loose?: boolean
+): void;
+
+export = defineDataProperty;