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/helpers/timeConstants.js

@@ -0,0 +1,19 @@
+'use strict';
+
+var HoursPerDay = 24;
+var MinutesPerHour = 60;
+var SecondsPerMinute = 60;
+var msPerSecond = 1e3;
+var msPerMinute = msPerSecond * SecondsPerMinute;
+var msPerHour = msPerMinute * MinutesPerHour;
+var msPerDay = 86400000;
+
+module.exports = {
+	HoursPerDay: HoursPerDay,
+	MinutesPerHour: MinutesPerHour,
+	SecondsPerMinute: SecondsPerMinute,
+	msPerSecond: msPerSecond,
+	msPerMinute: msPerMinute,
+	msPerHour: msPerHour,
+	msPerDay: msPerDay
+};