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-weakref/test/index.js

@@ -0,0 +1,26 @@
+'use strict';
+
+var test = require('tape');
+var inspect = require('object-inspect');
+var forEach = require('for-each');
+
+var isWeakRef = require('..');
+
+test('isWeakRef', function (t) {
+	t.equal(typeof isWeakRef, 'function', 'is a function');
+
+	var nonWeakRefs = [undefined, null, true, false, 42, 0, Infinity, NaN, /a/g, function () {}, {}, []];
+	forEach(nonWeakRefs, function (nonWeakRef) {
+		t.equal(isWeakRef(nonWeakRef), false, inspect(nonWeakRef) + ' is not a WeakRef');
+	});
+
+	t.test('actual WeakRefs', { skip: typeof WeakRef === 'undefined' }, function (st) {
+		var ref = new WeakRef({});
+
+		st.equal(isWeakRef(ref), true, inspect(ref) + ' is a WeakRef');
+
+		st.end();
+	});
+
+	t.end();
+});