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/has.js

@@ -0,0 +1,15 @@
+'use strict';
+
+var inspect = require('../');
+var test = require('tape');
+var mockProperty = require('mock-property');
+
+test('when Object#hasOwnProperty is deleted', function (t) {
+    t.plan(1);
+    var arr = [1, , 3]; // eslint-disable-line no-sparse-arrays
+
+    t.teardown(mockProperty(Array.prototype, 1, { value: 2 })); // this is needed to account for "in" vs "hasOwnProperty"
+    t.teardown(mockProperty(Object.prototype, 'hasOwnProperty', { 'delete': true }));
+
+    t.equal(inspect(arr), '[ 1, , 3 ]');
+});