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/index.js

@@ -0,0 +1,17 @@
+'use strict';
+
+var assign = require('../');
+var test = require('tape');
+var runTests = require('./tests');
+
+test('as a function', function (t) {
+	t.test('bad array/this value', function (st) {
+		st['throws'](function () { assign(undefined); }, TypeError, 'undefined is not an object');
+		st['throws'](function () { assign(null); }, TypeError, 'null is not an object');
+		st.end();
+	});
+
+	runTests(assign, t);
+
+	t.end();
+});