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/es-abstract/2025/Day.js

@@ -0,0 +1,11 @@
+'use strict';
+
+var floor = require('./floor');
+
+var msPerDay = require('../helpers/timeConstants').msPerDay;
+
+// https://262.ecma-international.org/5.1/#sec-15.9.1.2
+
+module.exports = function Day(t) {
+	return floor(t / msPerDay);
+};