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/@babel/runtime/helpers/esm/toPrimitive.js

@@ -0,0 +1,12 @@
+import _typeof from "./typeof.js";
+function toPrimitive(t, r) {
+  if ("object" != _typeof(t) || !t) return t;
+  var e = t[Symbol.toPrimitive];
+  if (void 0 !== e) {
+    var i = e.call(t, r || "default");
+    if ("object" != _typeof(i)) return i;
+    throw new TypeError("@@toPrimitive must return a primitive value.");
+  }
+  return ("string" === r ? String : Number)(t);
+}
+export { toPrimitive as default };