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-inspect/test/undef.js

@@ -0,0 +1,12 @@
+var test = require('tape');
+var inspect = require('../');
+
+var obj = { a: 1, b: [3, 4, undefined, null], c: undefined, d: null };
+
+test('undef and null', function (t) {
+    t.plan(1);
+    t.equal(
+        inspect(obj),
+        '{ a: 1, b: [ 3, 4, undefined, null ], c: undefined, d: null }'
+    );
+});