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/fast-json-stable-stringify/example/key_cmp.js

@@ -0,0 +1,7 @@
+var stringify = require('../');
+
+var obj = { c: 8, b: [{z:6,y:5,x:4},7], a: 3 };
+var s = stringify(obj, function (a, b) {
+    return a.key < b.key ? 1 : -1;
+});
+console.log(s);