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/has-bigints/test/index.js

@@ -0,0 +1,44 @@
+'use strict';
+
+var test = require('tape');
+var hasBigInts = require('..');
+
+test('interface', function (t) {
+	t.equal(typeof hasBigInts, 'function', 'is a function');
+	t.equal(typeof hasBigInts(), 'boolean', 'returns a boolean');
+	t.end();
+});
+
+test('BigInts are supported', { skip: !hasBigInts() }, function (t) {
+	t.equal(typeof BigInt, 'function', 'global BigInt is a function');
+	if (typeof BigInt !== 'function') {
+		return;
+	}
+
+	t.equal(BigInt(42), BigInt(42), '42n === 42n');
+	t['throws'](
+		function () { BigInt(NaN); },
+		RangeError,
+		'NaN is not an integer; BigInt(NaN) throws'
+	);
+
+	t['throws'](
+		function () { BigInt(Infinity); },
+		RangeError,
+		'Infinity is not an integer; BigInt(Infinity) throws'
+	);
+
+	t['throws'](
+		function () { BigInt(1.1); },
+		RangeError,
+		'1.1 is not an integer; BigInt(1.1) throws'
+	);
+
+	t.end();
+});
+
+test('BigInts are not supported', { skip: hasBigInts() }, function (t) {
+	t.equal(typeof BigInt, 'undefined', 'global BigInt is undefined');
+
+	t.end();
+});