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/vite-plugin-pwa/vanillajs.d.ts

@@ -0,0 +1,13 @@
+declare module 'virtual:pwa-register' {
+  import type { RegisterSWOptions } from 'vite-plugin-pwa/types'
+
+  export type { RegisterSWOptions }
+
+  /**
+   * Registers the service worker returning a callback to reload the current page when an update is found.
+   *
+   * @param options the options to register the service worker.
+   * @return (reloadPage?: boolean) => Promise<void> From version 0.13.2+ `reloadPage` param is not used anymore.
+   */
+  export function registerSW(options?: RegisterSWOptions): (reloadPage?: boolean) => Promise<void>
+}