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/copy-workbox-libraries.d.ts

@@ -0,0 +1,20 @@
+/**
+ * This copies over a set of runtime libraries used by Workbox into a
+ * local directory, which should be deployed alongside your service worker file.
+ *
+ * As an alternative to deploying these local copies, you could instead use
+ * Workbox from its official CDN URL.
+ *
+ * This method is exposed for the benefit of developers using
+ * {@link workbox-build.injectManifest} who would
+ * prefer not to use the CDN copies of Workbox. Developers using
+ * {@link workbox-build.generateSW} don't need to
+ * explicitly call this method.
+ *
+ * @param {string} destDirectory The path to the parent directory under which
+ * the new directory of libraries will be created.
+ * @return {Promise<string>} The name of the newly created directory.
+ *
+ * @alias workbox-build.copyWorkboxLibraries
+ */
+export declare function copyWorkboxLibraries(destDirectory: string): Promise<string>;