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/@isaacs/cliui/dist/esm/emoji-regex/index.d.ts

@@ -0,0 +1,2 @@
+export declare const emojiRegex: () => RegExp;
+//# sourceMappingURL=index.d.ts.map

frontend/node_modules/@isaacs/cliui/dist/esm/emoji-regex/index.d.ts.map

@@ -0,0 +1 @@
+{"version":3,"file":"index.d.ts","sourceRoot":"","sources":["../../../src/emoji-regex/index.ts"],"names":[],"mappings":"AACA,eAAO,MAAM,UAAU,cAC2we,CAAC"}