feat: add web UI and Docker setup for ZPLify

- HTTP server wrapping zplgfa library (port 4543)
- File upload with drag-and-drop, width/height resize, encoding selection, image edits
- /convert endpoint returns ZPL output; /preview returns image dimensions
- Dockerfile (multi-stage, Alpine) and docker-compose.yml

Dockerfile

@@ -0,0 +1,24 @@
+FROM golang:1.22-alpine AS builder
+
+WORKDIR /src
+
+# Copy module files and download deps
+COPY go.mod go.sum ./
+RUN go mod download
+
+# Copy source
+COPY . .
+
+# Build the web server
+RUN CGO_ENABLED=0 GOOS=linux go build -ldflags="-s -w" -o /zplgfa-web ./cmd/web
+
+# ─── Final stage ────────────────────────────────────────────────────────────
+FROM alpine:3.20
+
+RUN apk add --no-cache ca-certificates
+
+COPY --from=builder /zplgfa-web /usr/local/bin/zplgfa-web
+
+EXPOSE 4543
+
+ENTRYPOINT ["/usr/local/bin/zplgfa-web"]