Add local development environment with Devcontainers

[Smartdevs17]

Description

Setting up the development environment requires manual installation of multiple tools (Rust, Node, Soroban CLI). Create Devcontainer configuration for one-click setup.

Acceptance Criteria

• Devcontainer configuration with all dependencies
• Post-create setup script for contract compilation
• VS Code extension recommendations
• Docker Compose for backend services (Postgres, Redis)
• Documentation for Devcontainer usage
• CI check for Devcontainer build

Technical Scope

• .devcontainer/
• docker-compose.yml
• Edge: M1/ARM compatibility, Windows path mounting issues