Improve packages/desktop README with environment setup instructions #8488

The desktop package README lacks crucial environment setup information, specifically the RUST_TARGET environment variable requirement that causes the "RUST_TARGET not set" error.

Problem:

• New developers encounter RUST_TARGET not set error when running development commands
• No clear documentation of required environment variables
• Missing platform-specific setup instructions
• No troubleshooting section for common issues

Solution:

Enhance the README with:

• Environment variable setup requirements
• Platform-specific target configurations (macOS/Windows/Linux)
• Troubleshooting section for common errors
• Prerequisites and dependency installation details

Additional Context:

• This improves developer onboarding and reduces friction when setting up the desktop development environment.