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/lucide-react/dist/esm/icons/send-to-back.mjs

@@ -0,0 +1,19 @@
+/**
+ * @license lucide-react v1.24.0 - ISC
+ *
+ * This source code is licensed under the ISC license.
+ * See the LICENSE file in the root directory of this source tree.
+ */
+
+import createLucideIcon from '../createLucideIcon.mjs';
+
+const __iconNode = [
+  ["rect", { x: "14", y: "14", width: "8", height: "8", rx: "2", key: "1b0bso" }],
+  ["rect", { x: "2", y: "2", width: "8", height: "8", rx: "2", key: "1x09vl" }],
+  ["path", { d: "M7 14v1a2 2 0 0 0 2 2h1", key: "pao6x6" }],
+  ["path", { d: "M14 7h1a2 2 0 0 1 2 2v1", key: "19tdru" }]
+];
+const SendToBack = createLucideIcon("send-to-back", __iconNode);
+
+export { __iconNode, SendToBack as default };
+//# sourceMappingURL=send-to-back.mjs.map