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/2021/Number/remainder.js

@@ -0,0 +1,29 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+var isFinite = require('math-intrinsics/isFinite');
+
+var isNaN = require('../../helpers/isNaN');
+
+// https://262.ecma-international.org/12.0/#sec-numeric-types-number-remainder
+
+module.exports = function NumberRemainder(n, d) {
+	if (typeof n !== 'number' || typeof d !== 'number') {
+		throw new $TypeError('Assertion failed: `n` and `d` arguments must be Numbers');
+	}
+
+	// If either operand is NaN, the result is NaN.
+	// If the dividend is an infinity, or the divisor is a zero, or both, the result is NaN.
+	if (isNaN(n) || isNaN(d) || !isFinite(n) || d === 0) {
+		return NaN;
+	}
+
+	// If the dividend is finite and the divisor is an infinity, the result equals the dividend.
+	// If the dividend is a zero and the divisor is nonzero and finite, the result is the same as the dividend.
+	if (!isFinite(d) || n === 0) {
+		return n;
+	}
+
+	// In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved…
+	return n % d;
+};