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/fast-json-stable-stringify/test/to-json.js

@@ -0,0 +1,22 @@
+'use strict';
+
+var test = require('tape');
+var stringify = require('../');
+
+test('toJSON function', function (t) {
+    t.plan(1);
+    var obj = { one: 1, two: 2, toJSON: function() { return { one: 1 }; } };
+    t.equal(stringify(obj), '{"one":1}' );
+});
+
+test('toJSON returns string', function (t) {
+    t.plan(1);
+    var obj = { one: 1, two: 2, toJSON: function() { return 'one'; } };
+    t.equal(stringify(obj), '"one"');
+});
+
+test('toJSON returns array', function (t) {
+    t.plan(1);
+    var obj = { one: 1, two: 2, toJSON: function() { return ['one']; } };
+    t.equal(stringify(obj), '["one"]');
+});