fix(hf): revert to standard demo.launch for ZeroGPU compatibility and add base64 API endpoint

fix: add /custom/upload FastAPI endpoint + fix Gradio file URL format per official docs

fix: send base64 ImageData directly to Gradio 6 (no /upload - not available in SSR mode)

fix: rewrite API route - upload to HF /upload first, then call /call/remove_bg, return base64

feat: add Next.js frontend for Vercel + enable Gradio API on HuggingFace