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-cacheable-response/CacheableResponsePlugin.d.ts

@@ -0,0 +1,36 @@
+import { WorkboxPlugin } from 'workbox-core/types.js';
+import { CacheableResponseOptions } from './CacheableResponse.js';
+import './_version.js';
+/**
+ * A class implementing the `cacheWillUpdate` lifecycle callback. This makes it
+ * easier to add in cacheability checks to requests made via Workbox's built-in
+ * strategies.
+ *
+ * @memberof workbox-cacheable-response
+ */
+declare class CacheableResponsePlugin implements WorkboxPlugin {
+    private readonly _cacheableResponse;
+    /**
+     * To construct a new CacheableResponsePlugin instance you must provide at
+     * least one of the `config` properties.
+     *
+     * If both `statuses` and `headers` are specified, then both conditions must
+     * be met for the `Response` to be considered cacheable.
+     *
+     * @param {Object} config
+     * @param {Array<number>} [config.statuses] One or more status codes that a
+     * `Response` can have and be considered cacheable.
+     * @param {Object<string,string>} [config.headers] A mapping of header names
+     * and expected values that a `Response` can have and be considered cacheable.
+     * If multiple headers are provided, only one needs to be present.
+     */
+    constructor(config: CacheableResponseOptions);
+    /**
+     * @param {Object} options
+     * @param {Response} options.response
+     * @return {Response|null}
+     * @private
+     */
+    cacheWillUpdate: WorkboxPlugin['cacheWillUpdate'];
+}
+export { CacheableResponsePlugin };