Restructure project to support multiple templates via Nix flakes and omnix.
Users can now choose between vanilla JavaScript or Next.js TypeScript
templates during initialization.
- Move existing template to templates/vanilla/
- Add templates/nextjs/ with App Router, TypeScript, and React 19
- Update flake.nix for multi-template outputs
- Add TypeScript support to shared devshell
- Preserve FHS compatibility for both templates
- Update README with comprehensive template comparison and usage
