Refactor config, auth, and storage modules with runtime validation and encryption

- Centralized and refactored configuration management (`config`) with runtime validation for environment variables.
- Introduced utilities for secure token storage, including AES-GCM encryption and fallback handling.
- Enhanced `authStore` state management with token validation, secure persistence, and initialization from storage.
- Modularized authentication utilities and updated export structure for better maintainability.
- Improved error handling, input validation, and added detailed comments for enhanced clarity.

frontend/src/stores/index.ts

@@ -1,4 +1,4 @@
 // Zustand stores
 // Examples: authStore, uiStore, etc.
 
-export { useAuthStore } from './authStore';
+export { useAuthStore, initializeAuth, type User } from './authStore';