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-precaching/utils/PrecacheCacheKeyPlugin.d.ts

@@ -0,0 +1,17 @@
+import { WorkboxPlugin } from 'workbox-core/types.js';
+import { PrecacheController } from '../PrecacheController.js';
+import '../_version.js';
+/**
+ * A plugin, designed to be used with PrecacheController, to translate URLs into
+ * the corresponding cache key, based on the current revision info.
+ *
+ * @private
+ */
+declare class PrecacheCacheKeyPlugin implements WorkboxPlugin {
+    private readonly _precacheController;
+    constructor({ precacheController }: {
+        precacheController: PrecacheController;
+    });
+    cacheKeyWillBeUsed: WorkboxPlugin['cacheKeyWillBeUsed'];
+}
+export { PrecacheCacheKeyPlugin };