feat: add web playground with Docker sandbox

- playground/backend/main.go — Go HTTP server with /compile endpoint
- playground/frontend/index.html — Monaco Editor with 8 code examples
- playground/sandbox/Dockerfile — isolated Docker container with buxc2
- playground/docker-compose.yml — orchestrates backend + sandbox
- playground/Makefile — build and run targets
- playground/README.md — deployment guide for VPS with nginx + SSL

playground/backend/Dockerfile

@@ -0,0 +1,18 @@
+FROM golang:1.22-alpine AS builder
+
+WORKDIR /app
+COPY main.go .
+RUN go mod init bux-playground 2>/dev/null || true
+RUN go build -o playground-server main.go
+
+FROM alpine:latest
+
+RUN apk add --no-cache ca-certificates docker-cli
+
+WORKDIR /app
+COPY --from=builder /app/playground-server .
+COPY frontend ./frontend
+
+EXPOSE 8080
+
+CMD ["./playground-server"]