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/math-intrinsics/constants/maxValue.js

@@ -0,0 +1,5 @@
+'use strict';
+
+/** @type {import('./maxValue')}  */
+// eslint-disable-next-line no-extra-parens
+module.exports = /** @type {import('./maxValue')}  */ (Number.MAX_VALUE) || 1.7976931348623157e+308;