Add Flutter configuration for .devcontainers / GitHub Codespaces

**Type of Issue**  
- [x] New Feature Implementation
- [ ] Addition / Enhancement / Improvement

**Type of Work**  
- [ ] Screen  
- [ ] Widget/Component  
- [ ] Business Logic (Service, Repository, BloC)  
- [ ] Backend Service
- [x] DevOps
- [ ] Documentation

**Description**  
Add Flutter configuration for .devcontainers / GitHub Codespaces