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-date-object/.eslintrc

@@ -0,0 +1,9 @@
+{
+	"root": true,
+
+	"extends": "@ljharb",
+
+	"rules": {
+		//"max-statements": [2, 12]
+	}
+}