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-build/build/get-manifest.js

@@ -0,0 +1,35 @@
+"use strict";
+/*
+  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.
+*/
+Object.defineProperty(exports, "__esModule", { value: true });
+exports.getManifest = void 0;
+const get_file_manifest_entries_1 = require("./lib/get-file-manifest-entries");
+const validate_options_1 = require("./lib/validate-options");
+/**
+ * This method returns a list of URLs to precache, referred to as a "precache
+ * manifest", along with details about the number of entries and their size,
+ * based on the options you provide.
+ *
+ * ```
+ * // The following lists some common options; see the rest of the documentation
+ * // for the full set of options and defaults.
+ * const {count, manifestEntries, size, warnings} = await getManifest({
+ *   dontCacheBustURLsMatching: [new RegExp('...')],
+ *   globDirectory: '...',
+ *   globPatterns: ['...', '...'],
+ *   maximumFileSizeToCacheInBytes: ...,
+ * });
+ * ```
+ *
+ * @memberof workbox-build
+ */
+async function getManifest(config) {
+    const options = (0, validate_options_1.validateGetManifestOptions)(config);
+    return await (0, get_file_manifest_entries_1.getFileManifestEntries)(options);
+}
+exports.getManifest = getManifest;