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/PrecacheFallbackPlugin.js

@@ -0,0 +1,47 @@
+/*
+  Copyright 2020 Google LLC
+
+  Use of this source code is governed by an MIT-style
+  license that can be found in the LICENSE file or at
+  https://opensource.org/licenses/MIT.
+*/
+import { getOrCreatePrecacheController } from './utils/getOrCreatePrecacheController.js';
+import './_version.js';
+/**
+ * `PrecacheFallbackPlugin` allows you to specify an "offline fallback"
+ * response to be used when a given strategy is unable to generate a response.
+ *
+ * It does this by intercepting the `handlerDidError` plugin callback
+ * and returning a precached response, taking the expected revision parameter
+ * into account automatically.
+ *
+ * Unless you explicitly pass in a `PrecacheController` instance to the
+ * constructor, the default instance will be used. Generally speaking, most
+ * developers will end up using the default.
+ *
+ * @memberof workbox-precaching
+ */
+class PrecacheFallbackPlugin {
+    /**
+     * Constructs a new PrecacheFallbackPlugin with the associated fallbackURL.
+     *
+     * @param {Object} config
+     * @param {string} config.fallbackURL A precached URL to use as the fallback
+     *     if the associated strategy can't generate a response.
+     * @param {PrecacheController} [config.precacheController] An optional
+     *     PrecacheController instance. If not provided, the default
+     *     PrecacheController will be used.
+     */
+    constructor({ fallbackURL, precacheController, }) {
+        /**
+         * @return {Promise<Response>} The precache response for the fallback URL.
+         *
+         * @private
+         */
+        this.handlerDidError = () => this._precacheController.matchPrecache(this._fallbackURL);
+        this._fallbackURL = fallbackURL;
+        this._precacheController =
+            precacheController || getOrCreatePrecacheController();
+    }
+}
+export { PrecacheFallbackPlugin };