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/core-js-compat/index.d.ts

@@ -0,0 +1,27 @@
+import type compat from './compat'
+import type getModulesListForTargetVersion from './get-modules-list-for-target-version';
+import type { ModuleName, Target, TargetVersion } from './shared'
+
+type CompatData = {
+  [module: ModuleName]: {
+    [target in Target]?: TargetVersion
+  }
+};
+
+declare const ExportedCompatObject: typeof compat & {
+  compat: typeof compat,
+
+  /** The subset of modules which available in the passed `core-js` version */
+  getModulesListForTargetVersion: typeof getModulesListForTargetVersion,
+
+  /** Full list compatibility data */
+  data: CompatData,
+
+  /** map of modules by `core-js` entry points */
+  entries: {[entry_point: string]: readonly ModuleName[]},
+
+  /** Full list of modules */
+  modules: readonly ModuleName[]
+}
+
+export = ExportedCompatObject