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/safe-regex-test/test/index.js

@@ -0,0 +1,41 @@
+'use strict';
+
+var test = require('tape');
+var v = require('es-value-fixtures');
+var forEach = require('for-each');
+var inspect = require('object-inspect');
+
+var regexTester = require('../');
+
+test('regex tester', function (t) {
+	t.equal(typeof regexTester, 'function', 'is a function');
+
+	t.test('non-regexes', function (st) {
+		forEach([].concat(
+			// @ts-expect-error TS sucks with concat
+			v.primitives,
+			v.objects
+		), function (val) {
+			st['throws'](
+				function () { regexTester(val); },
+				TypeError,
+				inspect(val) + ' is not a regex'
+			);
+		});
+
+		st.end();
+	});
+
+	t.test('regexes', function (st) {
+		var tester = regexTester(/a/);
+
+		st.equal(typeof tester, 'function', 'returns a function');
+		st.equal(tester('a'), true, 'returns true for a match');
+		st.equal(tester('b'), false, 'returns false for a non-match');
+		st.equal(tester('a'), true, 'returns true for a match again');
+
+		st.end();
+	});
+
+	t.end();
+});