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-range-requests/RangeRequestsPlugin.js

@@ -0,0 +1,44 @@
+/*
+  Copyright 2018 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 { createPartialResponse } from './createPartialResponse.js';
+import './_version.js';
+/**
+ * The range request plugin makes it easy for a request with a 'Range' header to
+ * be fulfilled by a cached response.
+ *
+ * It does this by intercepting the `cachedResponseWillBeUsed` plugin callback
+ * and returning the appropriate subset of the cached response body.
+ *
+ * @memberof workbox-range-requests
+ */
+class RangeRequestsPlugin {
+    constructor() {
+        /**
+         * @param {Object} options
+         * @param {Request} options.request The original request, which may or may not
+         * contain a Range: header.
+         * @param {Response} options.cachedResponse The complete cached response.
+         * @return {Promise<Response>} If request contains a 'Range' header, then a
+         * new response with status 206 whose body is a subset of `cachedResponse` is
+         * returned. Otherwise, `cachedResponse` is returned as-is.
+         *
+         * @private
+         */
+        this.cachedResponseWillBeUsed = async ({ request, cachedResponse, }) => {
+            // Only return a sliced response if there's something valid in the cache,
+            // and there's a Range: header in the request.
+            if (cachedResponse && request.headers.has('range')) {
+                return await createPartialResponse(request, cachedResponse);
+            }
+            // If there was no Range: header, or if cachedResponse wasn't valid, just
+            // pass it through as-is.
+            return cachedResponse;
+        };
+    }
+}
+export { RangeRequestsPlugin };