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/brace-expansion/dist/esm/index.d.ts

@@ -0,0 +1,6 @@
+export declare const EXPANSION_MAX = 100000;
+export type BraceExpansionOptions = {
+    max?: number;
+};
+export declare function expand(str: string, options?: BraceExpansionOptions): string[];
+//# sourceMappingURL=index.d.ts.map