feat: add local development setup with Docker and Next.js integration

[dhulke]

I tried running this project locally without using your production api gateway and had some issues, so I made some changes to allow it to run fully locally. Hope this is useful. Excellent project by the way, thank you.

Below is a summary of what this PR adds.

• Add start-local.sh script for coordinated Lambda + Next.js startup
• Support configurable ports for both services
• Add Next.js API proxy route to avoid CORS issues
• Enable switching between local and production APIs
• Update README with comprehensive local development guide
• Add proper cleanup on Ctrl+C for Docker containers