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/2019/MakeDay.js

@@ -0,0 +1,33 @@
+'use strict';
+
+var GetIntrinsic = require('get-intrinsic');
+
+var $DateUTC = GetIntrinsic('%Date.UTC%');
+
+var $isFinite = require('math-intrinsics/isFinite');
+
+var DateFromTime = require('./DateFromTime');
+var Day = require('./Day');
+var floor = require('./floor');
+var modulo = require('./modulo');
+var MonthFromTime = require('./MonthFromTime');
+var ToInteger = require('./ToInteger');
+var YearFromTime = require('./YearFromTime');
+
+// https://262.ecma-international.org/5.1/#sec-15.9.1.12
+
+module.exports = function MakeDay(year, month, date) {
+	if (!$isFinite(year) || !$isFinite(month) || !$isFinite(date)) {
+		return NaN;
+	}
+	var y = ToInteger(year);
+	var m = ToInteger(month);
+	var dt = ToInteger(date);
+	var ym = y + floor(m / 12);
+	var mn = modulo(m, 12);
+	var t = $DateUTC(ym, mn, 1);
+	if (YearFromTime(t) !== ym || MonthFromTime(t) !== mn || DateFromTime(t) !== 1) {
+		return NaN;
+	}
+	return Day(t) + dt - 1;
+};