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/es-errors/test/index.js

@@ -0,0 +1,19 @@
+'use strict';
+
+var test = require('tape');
+
+var E = require('../');
+var R = require('../range');
+var Ref = require('../ref');
+var S = require('../syntax');
+var T = require('../type');
+
+test('errors', function (t) {
+	t.equal(E, Error);
+	t.equal(R, RangeError);
+	t.equal(Ref, ReferenceError);
+	t.equal(S, SyntaxError);
+	t.equal(T, TypeError);
+
+	t.end();
+});