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-weakset/test/index.js

@@ -0,0 +1,59 @@
+'use strict';
+
+var test = require('tape');
+var debug = require('object-inspect');
+var forEach = require('for-each');
+
+var isWeakSet = require('..');
+
+test('non-collections', function (t) {
+	forEach([
+		null,
+		undefined,
+		true,
+		false,
+		42,
+		0,
+		-0,
+		NaN,
+		Infinity,
+		'',
+		'foo',
+		/a/g,
+		[],
+		{},
+		function () {}
+	], function (nonCollection) {
+		t.equal(isWeakSet(nonCollection), false, debug(nonCollection) + ' is not a WeakSet');
+	});
+
+	t.end();
+});
+
+test('Maps', { skip: typeof Map !== 'function' }, function (t) {
+	var m = new Map();
+	t.equal(isWeakSet(m), false, debug(m) + ' is not a WeakSet');
+
+	t.end();
+});
+
+test('Sets', { skip: typeof Set !== 'function' }, function (t) {
+	var s = new Set();
+	t.equal(isWeakSet(s), false, debug(s) + ' is not a WeakSet');
+
+	t.end();
+});
+
+test('WeakMaps', { skip: typeof WeakMap !== 'function' }, function (t) {
+	var wm = new WeakMap();
+	t.equal(isWeakSet(wm), false, debug(wm) + ' is not a WeakSet');
+
+	t.end();
+});
+
+test('WeakSets', { skip: typeof WeakSet !== 'function' }, function (t) {
+	var ws = new WeakSet();
+	t.equal(isWeakSet(ws), true, debug(ws) + ' is a WeakSet');
+
+	t.end();
+});