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.d.ts

@@ -0,0 +1,12 @@
+/**
+ * This is a browser shim that provides the same functional interface
+ * as the main node export, but it does nothing.
+ * @module
+ */
+import type { Handler } from './index.js';
+export declare const onExit: (cb: Handler, opts: {
+    alwaysLast?: boolean;
+}) => () => void;
+export declare const load: () => void;
+export declare const unload: () => void;
+//# sourceMappingURL=browser.d.ts.map