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-recipes/offlineFallback.d.ts

@@ -0,0 +1,18 @@
+import './_version.js';
+export interface OfflineFallbackOptions {
+    pageFallback?: string;
+    imageFallback?: string;
+    fontFallback?: string;
+}
+/**
+ * An implementation of the [comprehensive fallbacks recipe]{@link https://developers.google.com/web/tools/workbox/guides/advanced-recipes#comprehensive_fallbacks}. Be sure to include the fallbacks in your precache injection
+ *
+ * @memberof workbox-recipes
+ *
+ * @param {Object} [options]
+ * @param {string} [options.pageFallback] Precache name to match for pag fallbacks. Defaults to offline.html
+ * @param {string} [options.imageFallback] Precache name to match for image fallbacks.
+ * @param {string} [options.fontFallback] Precache name to match for font fallbacks.
+ */
+declare function offlineFallback(options?: OfflineFallbackOptions): void;
+export { offlineFallback };