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/deepmerge/index.d.ts

@@ -0,0 +1,20 @@
+declare function deepmerge<T>(x: Partial<T>, y: Partial<T>, options?: deepmerge.Options): T;
+declare function deepmerge<T1, T2>(x: Partial<T1>, y: Partial<T2>, options?: deepmerge.Options): T1 & T2;
+
+declare namespace deepmerge {
+	export interface Options {
+		arrayMerge?(target: any[], source: any[], options?: ArrayMergeOptions): any[];
+		clone?: boolean;
+		customMerge?: (key: string, options?: Options) => ((x: any, y: any) => any) | undefined;
+		isMergeableObject?(value: object): boolean;
+	}
+	export interface ArrayMergeOptions {
+		isMergeableObject(value: object): boolean;
+		cloneUnlessOtherwiseSpecified(value: object, options?: Options): object;
+	}
+
+	export function all (objects: object[], options?: Options): object;
+	export function all<T> (objects: Partial<T>[], options?: Options): T;
+}
+
+export = deepmerge;