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-navigation-preload/disable.js

@@ -0,0 +1,32 @@
+/*
+  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 { logger } from 'workbox-core/_private/logger.js';
+import { isSupported } from './isSupported.js';
+import './_version.js';
+/**
+ * If the browser supports Navigation Preload, then this will disable it.
+ *
+ * @memberof workbox-navigation-preload
+ */
+function disable() {
+    if (isSupported()) {
+        self.addEventListener('activate', (event) => {
+            event.waitUntil(self.registration.navigationPreload.disable().then(() => {
+                if (process.env.NODE_ENV !== 'production') {
+                    logger.log(`Navigation preload is disabled.`);
+                }
+            }));
+        });
+    }
+    else {
+        if (process.env.NODE_ENV !== 'production') {
+            logger.log(`Navigation preload is not supported in this browser.`);
+        }
+    }
+}
+export { disable };