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/utils/PrecacheInstallReportPlugin.js

@@ -0,0 +1,44 @@
+/*
+  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 '../_version.js';
+/**
+ * A plugin, designed to be used with PrecacheController, to determine the
+ * of assets that were updated (or not updated) during the install event.
+ *
+ * @private
+ */
+class PrecacheInstallReportPlugin {
+    constructor() {
+        this.updatedURLs = [];
+        this.notUpdatedURLs = [];
+        this.handlerWillStart = async ({ request, state, }) => {
+            // TODO: `state` should never be undefined...
+            if (state) {
+                state.originalRequest = request;
+            }
+        };
+        this.cachedResponseWillBeUsed = async ({ event, state, cachedResponse, }) => {
+            if (event.type === 'install') {
+                if (state &&
+                    state.originalRequest &&
+                    state.originalRequest instanceof Request) {
+                    // TODO: `state` should never be undefined...
+                    const url = state.originalRequest.url;
+                    if (cachedResponse) {
+                        this.notUpdatedURLs.push(url);
+                    }
+                    else {
+                        this.updatedURLs.push(url);
+                    }
+                }
+            }
+            return cachedResponse;
+        };
+    }
+}
+export { PrecacheInstallReportPlugin };