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/is-async-function/test/uglified.js

@@ -0,0 +1,9 @@
+'use strict';
+
+// @ts-expect-error
+require('uglify-register/api').register({
+	exclude: [/\/node_modules\//, /\/test\//],
+	uglify: { mangle: true }
+});
+
+require('./');