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/balanced-match/dist/esm/index.d.ts

@@ -0,0 +1,9 @@
+export declare const balanced: (a: string | RegExp, b: string | RegExp, str: string) => false | {
+    start: number;
+    end: number;
+    pre: string;
+    body: string;
+    post: string;
+} | undefined;
+export declare const range: (a: string, b: string, str: string) => undefined | [number, number];
+//# sourceMappingURL=index.d.ts.map