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/lib/runtime/re2.ts

@@ -0,0 +1,6 @@
+import * as re2 from "re2"
+
+type Re2 = typeof re2 & {code: string}
+;(re2 as Re2).code = 'require("ajv/dist/runtime/re2").default'
+
+export default re2 as Re2