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/ajv/dist/runtime/validation_error.d.ts

@@ -0,0 +1,7 @@
+import type { ErrorObject } from "../types";
+export default class ValidationError extends Error {
+    readonly errors: Partial<ErrorObject>[];
+    readonly ajv: true;
+    readonly validation: true;
+    constructor(errors: Partial<ErrorObject>[]);
+}