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/holes.js

@@ -0,0 +1,15 @@
+var test = require('tape');
+var inspect = require('../');
+
+var xs = ['a', 'b'];
+xs[5] = 'f';
+xs[7] = 'j';
+xs[8] = 'k';
+
+test('holes', function (t) {
+    t.plan(1);
+    t.equal(
+        inspect(xs),
+        "[ 'a', 'b', , , , 'f', , 'j', 'k' ]"
+    );
+});