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/object.assign/test/implementation.js

@@ -0,0 +1,19 @@
+'use strict';
+
+var implementation = require('../implementation');
+var callBind = require('call-bind');
+var test = require('tape');
+var hasStrictMode = require('has-strict-mode')();
+var runTests = require('./tests');
+
+test('as a function', function (t) {
+	t.test('bad first arg/receiver', { skip: !hasStrictMode }, function (st) {
+		st['throws'](function () { implementation(undefined); }, TypeError, 'undefined is not an object');
+		st['throws'](function () { implementation(null); }, TypeError, 'null is not an object');
+		st.end();
+	});
+
+	runTests(callBind(implementation, Object), t);
+
+	t.end();
+});