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/sliders-horizontal.mjs

@@ -0,0 +1,24 @@
+/**
+ * @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 = [
+  ["path", { d: "M10 5H3", key: "1qgfaw" }],
+  ["path", { d: "M12 19H3", key: "yhmn1j" }],
+  ["path", { d: "M14 3v4", key: "1sua03" }],
+  ["path", { d: "M16 17v4", key: "1q0r14" }],
+  ["path", { d: "M21 12h-9", key: "1o4lsq" }],
+  ["path", { d: "M21 19h-5", key: "1rlt1p" }],
+  ["path", { d: "M21 5h-7", key: "1oszz2" }],
+  ["path", { d: "M8 10v4", key: "tgpxqk" }],
+  ["path", { d: "M8 12H3", key: "a7s4jb" }]
+];
+const SlidersHorizontal = createLucideIcon("sliders-horizontal", __iconNode);
+
+export { __iconNode, SlidersHorizontal as default };
+//# sourceMappingURL=sliders-horizontal.mjs.map