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/Makefile

@@ -0,0 +1,55 @@
+# Bux Playground — Makefile
+# Run from project root: make -C playground <target>
+
+.PHONY: all build-sandbox build-backend run stop clean
+
+SANDBOX_IMAGE = bux-playground-sandbox
+BACKEND_IMAGE = bux-playground-backend
+PORT = 8080
+
+all: build-sandbox build-backend
+
+# Build the sandbox Docker image (needs buxc2 + lib + rt)
+build-sandbox:
+	@echo "Building sandbox image..."
+	@docker build \
+		-f sandbox/Dockerfile \
+		-t $(SANDBOX_IMAGE) \
+		..
+
+# Build the backend Go server
+build-backend:
+	@echo "Building backend image..."
+	@docker build \
+		-f backend/Dockerfile \
+		-t $(BACKEND_IMAGE) \
+		.
+
+# Run everything with docker-compose
+run:
+	@echo "Starting Bux Playground on http://localhost:$(PORT)"
+	@docker-compose up
+
+# Run in background
+run-detached:
+	@echo "Starting Bux Playground in background on http://localhost:$(PORT)"
+	@docker-compose up -d
+
+# Stop
+stop:
+	@docker-compose down
+
+# Clean images and containers
+clean:
+	@docker-compose down --rmi all -v
+	@docker rmi $(SANDBOX_IMAGE) $(BACKEND_IMAGE) 2>/dev/null || true
+
+# Development mode (local backend, no Docker sandbox)
+dev:
+	@echo "Starting backend in dev mode..."
+	cd backend && go run main.go
+
+# Quick test
+test:
+	@echo "Testing /health endpoint..."
+	@curl -s http://localhost:$(PORT)/health | jq .