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/module-registry.d.ts

@@ -0,0 +1,33 @@
+/**
+ * Class for keeping track of which Workbox modules are used by the generated
+ * service worker script.
+ *
+ * @private
+ */
+export declare class ModuleRegistry {
+    private readonly _modulesUsed;
+    /**
+     * @private
+     */
+    constructor();
+    /**
+     * @return {Array<string>} A list of all of the import statements that are
+     * needed for the modules being used.
+     * @private
+     */
+    getImportStatements(): Array<string>;
+    /**
+     * @param {string} pkg The workbox package that the module belongs to.
+     * @param {string} moduleName The name of the module to import.
+     * @return {string} The local variable name that corresponds to that module.
+     * @private
+     */
+    getLocalName(pkg: string, moduleName: string): string;
+    /**
+     * @param {string} pkg The workbox package that the module belongs to.
+     * @param {string} moduleName The name of the module to import.
+     * @return {string} The local variable name that corresponds to that module.
+     * @private
+     */
+    use(pkg: string, moduleName: string): string;
+}