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/lowbyte.js

@@ -0,0 +1,12 @@
+var test = require('tape');
+var inspect = require('../');
+
+var obj = { x: 'a\r\nb', y: '\x05! \x1f \x12' };
+
+test('interpolate low bytes', function (t) {
+    t.plan(1);
+    t.equal(
+        inspect(obj),
+        "{ x: 'a\\r\\nb', y: '\\x05! \\x1F \\x12' }"
+    );
+});