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-routing/utils/constants.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 '../_version.js';
+/**
+ * The default HTTP method, 'GET', used when there's no specific method
+ * configured for a route.
+ *
+ * @type {string}
+ *
+ * @private
+ */
+export const defaultMethod = 'GET';
+/**
+ * The list of valid HTTP methods associated with requests that could be routed.
+ *
+ * @type {Array<string>}
+ *
+ * @private
+ */
+export const validMethods = [
+    'DELETE',
+    'GET',
+    'HEAD',
+    'PATCH',
+    'POST',
+    'PUT',
+];