feat: Node.js server + editor save/load + keyboard controls

- Replace nginx with Node.js server that serves static files AND
  provides API endpoints for reading/writing maps.js directly
  (GET/PUT /api/maps). Creates .bak backup before each save.
- Editor: arrow keys to pan, +/- to zoom, Ctrl+S to save
- Editor: "Save" button writes maps.js directly on the server
- Editor: "Load" button reads and parses maps.js from server
- Editor: auto-loads from server on page open
- Dockerfile changed from nginx:alpine to node:20-alpine

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile

@@ -1,7 +1,11 @@
-FROM nginx:alpine
-COPY index.html /usr/share/nginx/html/
-COPY editor.html /usr/share/nginx/html/
-COPY css/ /usr/share/nginx/html/css/
-COPY js/ /usr/share/nginx/html/js/
-COPY assets/ /usr/share/nginx/html/assets/
+FROM node:20-alpine
+WORKDIR /app
+COPY server.js .
+RUN mkdir -p public
+COPY index.html public/
+COPY editor.html public/
+COPY css/ public/css/
+COPY js/ public/js/
+COPY assets/ public/assets/
 EXPOSE 80
+CMD ["node", "server.js"]