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/signal-exit/dist/cjs/browser.js

@@ -0,0 +1,10 @@
+"use strict";
+Object.defineProperty(exports, "__esModule", { value: true });
+exports.unload = exports.load = exports.onExit = void 0;
+const onExit = () => () => { };
+exports.onExit = onExit;
+const load = () => { };
+exports.load = load;
+const unload = () => { };
+exports.unload = unload;
+//# sourceMappingURL=browser.js.map