refactor: reorganize package exports under namespaced paths

- Prefix feature exports with `features/` (auth, admin, provider)
- Prefix shared exports with `shared/` (components, icons, lib, config, logger, rate-limit)
- Add new explicit exports for `shared/logger`, `shared/config`, and `shared/rate-limit`
- Update internal imports to use package self-referencing (`@zen/core/shared/*`) instead of relative paths

src/features/admin/actions.js

@@ -4,7 +4,7 @@
  * Exported separately from admin/index.js to avoid bundling
  * server-side code (which includes database imports) into client components.
  *
- * Usage: import { getDashboardStats } from '@zen/core/admin/actions';
+ * Usage: import { getDashboardStats } from '@zen/core/features/admin/actions';
  */
 
 export { getDashboardStats } from './actions/statsActions.js';