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/lib/replace-and-update-source-map.d.ts

@@ -0,0 +1,30 @@
+import { RawSourceMap } from 'source-map';
+/**
+ * Adapted from https://github.com/nsams/sourcemap-aware-replace, with modern
+ * JavaScript updates, along with additional properties copied from originalMap.
+ *
+ * @param {Object} options
+ * @param {string} options.jsFilename The name for the file whose contents
+ * correspond to originalSource.
+ * @param {Object} options.originalMap The sourcemap for originalSource,
+ * prior to any replacements.
+ * @param {string} options.originalSource The source code, prior to any
+ * replacements.
+ * @param {string} options.replaceString A string to swap in for searchString.
+ * @param {string} options.searchString A string in originalSource to replace.
+ * Only the first occurrence will be replaced.
+ * @return {{source: string, map: string}} An object containing both
+ * originalSource with the replacement applied, and the modified originalMap.
+ *
+ * @private
+ */
+export declare function replaceAndUpdateSourceMap({ jsFilename, originalMap, originalSource, replaceString, searchString, }: {
+    jsFilename: string;
+    originalMap: RawSourceMap;
+    originalSource: string;
+    replaceString: string;
+    searchString: string;
+}): Promise<{
+    map: string;
+    source: string;
+}>;