Implement Stellar Wallet Integration Module

# 🧠 Concept  
Add Stellar wallet connectivity support.

## ⚠️ Problem  
Developers cannot easily test integrated contract flows.

## 📁 Implementation Scope  
- `src/integrations/wallets/stellar/`

## ✅ Requirements  
- Support Freighter wallet integration  
- Enable transaction signing flows  

## 🎯 Acceptance Criteria  
- [ ] Wallet integration functional  