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/workbox-build/build/lib/bundle.d.ts

@@ -0,0 +1,9 @@
+import { GeneratePartial, RequiredSWDestPartial } from '../types';
+interface NameAndContents {
+    contents: string | Uint8Array;
+    name: string;
+}
+export declare function bundle({ babelPresetEnvTargets, inlineWorkboxRuntime, mode, sourcemap, swDest, unbundledCode, }: Omit<GeneratePartial, 'runtimeCaching'> & RequiredSWDestPartial & {
+    unbundledCode: string;
+}): Promise<Array<NameAndContents>>;
+export {};