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

docker-compose.yml

@@ -0,0 +1,6 @@
+services:
+  zplgfa:
+    build: .
+    ports:
+      - "4543:4543"
+    restart: unless-stopped