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/standalone/instance.d.ts

@@ -0,0 +1,12 @@
+import Ajv, { AnySchema, AnyValidateFunction, ErrorObject } from "../core";
+export default class AjvPack {
+    readonly ajv: Ajv;
+    errors?: ErrorObject[] | null;
+    constructor(ajv: Ajv);
+    validate(schemaKeyRef: AnySchema | string, data: unknown): boolean | Promise<unknown>;
+    compile<T = unknown>(schema: AnySchema, meta?: boolean): AnyValidateFunction<T>;
+    getSchema<T = unknown>(keyRef: string): AnyValidateFunction<T> | undefined;
+    private getStandalone;
+    addSchema(...args: Parameters<typeof Ajv.prototype.addSchema>): AjvPack;
+    addKeyword(...args: Parameters<typeof Ajv.prototype.addKeyword>): AjvPack;
+}