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/is-date-object/test/index.js

@@ -0,0 +1,38 @@
+'use strict';
+
+var test = require('tape');
+var isDate = require('../');
+var hasToStringTag = require('has-tostringtag/shams')();
+
+test('not Dates', function (t) {
+	// @ts-expect-error
+	t.notOk(isDate(), 'undefined is not Date');
+	t.notOk(isDate(null), 'null is not Date');
+	t.notOk(isDate(false), 'false is not Date');
+	t.notOk(isDate(true), 'true is not Date');
+	t.notOk(isDate(42), 'number is not Date');
+	t.notOk(isDate('foo'), 'string is not Date');
+	t.notOk(isDate([]), 'array is not Date');
+	t.notOk(isDate({}), 'object is not Date');
+	t.notOk(isDate(function () {}), 'function is not Date');
+	t.notOk(isDate(/a/g), 'regex literal is not Date');
+	t.notOk(isDate(new RegExp('a', 'g')), 'regex object is not Date');
+	t.end();
+});
+
+test('@@toStringTag', { skip: !hasToStringTag }, function (t) {
+	var realDate = new Date();
+	/** @type {{ toString(): unknown; valueOf(): unknown; [Symbol.toStringTag]?: string; }} */
+	var fakeDate = {
+		toString: function () { return String(realDate); },
+		valueOf: function () { return realDate.getTime(); }
+	};
+	fakeDate[Symbol.toStringTag] = 'Date';
+	t.notOk(isDate(fakeDate), 'fake Date with @@toStringTag "Date" is not Date');
+	t.end();
+});
+
+test('Dates', function (t) {
+	t.ok(isDate(new Date()), 'new Date() is Date');
+	t.end();
+});