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/circular.js

@@ -0,0 +1,16 @@
+var inspect = require('../');
+var test = require('tape');
+
+test('circular', function (t) {
+    t.plan(2);
+    var obj = { a: 1, b: [3, 4] };
+    obj.c = obj;
+    t.equal(inspect(obj), '{ a: 1, b: [ 3, 4 ], c: [Circular] }');
+
+    var double = {};
+    double.a = [double];
+    double.b = {};
+    double.b.inner = double.b;
+    double.b.obj = double;
+    t.equal(inspect(double), '{ a: [ [Circular] ], b: { inner: [Circular], obj: [Circular] } }');
+});