Implement Stellar Multi-Network Environment

# 🧠 Concept  
Support Testnet, Futurenet, and Mainnet environments.

## ⚠️ Problem  
Environment configuration is inconsistent across workflows.

## 📁 Implementation Scope  
- `config/stellar/`

## ✅ Requirements  
- Add network abstraction layer  
- Support environment switching  

## 🎯 Acceptance Criteria  
- [ ] Multi-network support implemented  