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/foreground-child/dist/commonjs/watchdog.d.ts

@@ -0,0 +1,10 @@
+import { ChildProcess } from 'child_process';
+/**
+ * Pass in a ChildProcess, and this will spawn a watchdog process that
+ * will make sure it exits if the parent does, thus preventing any
+ * dangling detached zombie processes.
+ *
+ * If the child ends before the parent, then the watchdog will terminate.
+ */
+export declare const watchdog: (child: ChildProcess) => ChildProcess;
+//# sourceMappingURL=watchdog.d.ts.map