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-strategies/CacheFirst.d.ts

@@ -0,0 +1,28 @@
+import { Strategy } from './Strategy.js';
+import { StrategyHandler } from './StrategyHandler.js';
+import './_version.js';
+/**
+ * An implementation of a [cache-first](https://developer.chrome.com/docs/workbox/caching-strategies-overview/#cache-first-falling-back-to-network)
+ * request strategy.
+ *
+ * A cache first strategy is useful for assets that have been revisioned,
+ * such as URLs like `/styles/example.a8f5f1.css`, since they
+ * can be cached for long periods of time.
+ *
+ * If the network request fails, and there is no cache match, this will throw
+ * a `WorkboxError` exception.
+ *
+ * @extends workbox-strategies.Strategy
+ * @memberof workbox-strategies
+ */
+declare class CacheFirst extends Strategy {
+    /**
+     * @private
+     * @param {Request|string} request A request to run this strategy for.
+     * @param {workbox-strategies.StrategyHandler} handler The event that
+     *     triggered the request.
+     * @return {Promise<Response>}
+     */
+    _handle(request: Request, handler: StrategyHandler): Promise<Response>;
+}
+export { CacheFirst };