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-core/cacheNames.d.ts

@@ -0,0 +1,24 @@
+import './_version.js';
+/**
+ * Get the current cache names and prefix/suffix used by Workbox.
+ *
+ * `cacheNames.precache` is used for precached assets,
+ * `cacheNames.googleAnalytics` is used by `workbox-google-analytics` to
+ * store `analytics.js`, and `cacheNames.runtime` is used for everything else.
+ *
+ * `cacheNames.prefix` can be used to retrieve just the current prefix value.
+ * `cacheNames.suffix` can be used to retrieve just the current suffix value.
+ *
+ * @return {Object} An object with `precache`, `runtime`, `prefix`, and
+ *     `googleAnalytics` properties.
+ *
+ * @memberof workbox-core
+ */
+declare const cacheNames: {
+    readonly googleAnalytics: string;
+    readonly precache: string;
+    readonly prefix: string;
+    readonly runtime: string;
+    readonly suffix: string;
+};
+export { cacheNames };