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-background-sync/BackgroundSyncPlugin.d.ts

@@ -0,0 +1,27 @@
+import { WorkboxPlugin } from 'workbox-core/types.js';
+import { QueueOptions } from './Queue.js';
+import './_version.js';
+/**
+ * A class implementing the `fetchDidFail` lifecycle callback. This makes it
+ * easier to add failed requests to a background sync Queue.
+ *
+ * @memberof workbox-background-sync
+ */
+declare class BackgroundSyncPlugin implements WorkboxPlugin {
+    private readonly _queue;
+    /**
+     * @param {string} name See the {@link workbox-background-sync.Queue}
+     *     documentation for parameter details.
+     * @param {Object} [options] See the
+     *     {@link workbox-background-sync.Queue} documentation for
+     *     parameter details.
+     */
+    constructor(name: string, options?: QueueOptions);
+    /**
+     * @param {Object} options
+     * @param {Request} options.request
+     * @private
+     */
+    fetchDidFail: WorkboxPlugin['fetchDidFail'];
+}
+export { BackgroundSyncPlugin };