Replace cross-env with a simple Node.js wrapper script that:
- Respects externally supplied PORT environment variable
- Falls back to 8502 if PORT is not set
- Works cross-platform without extra dependencies
- No runtime dependencies beyond Node.js itself
Changes:
- Add start-server.js wrapper script
- Update package.json to use wrapper
- Remove cross-env dependency
- Copy start-server.js in Dockerfile
This fixes both issues:
1. Preserves PORT fallback behavior (PORT can be overridden)
2. No extra runtime dependencies needed
