Add web service, PDF export, and Docker support

- Refactor conversion logic into reusable convert.js module
- Add HTTP server with upload UI, HTML/PDF download, and live preview
- Dockerfile with Chromium for server-side PDF rendering
- Local mermaid.min.js injection for offline diagram rendering

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -1,6 +1,5 @@
 FROM node:22-slim
 
-# Install Chrome dependencies
 RUN apt-get update && apt-get install -y --no-install-recommends \
     chromium \
     fonts-freefont-ttf \
@@ -12,8 +11,14 @@ WORKDIR /app
 COPY package.json package-lock.json ./
 RUN npm ci --omit=dev
 
-COPY md2html.js mermaid.min.js ./
+COPY convert.js server.js md2html.js mermaid.min.js ./
 
 ENV CHROME_PATH=/usr/bin/chromium
+ENV PORT=3000
 
-ENTRYPOINT ["node", "md2html.js"]
+EXPOSE 3000
+
+HEALTHCHECK --interval=30s --timeout=5s --retries=3 \
+  CMD node -e "require('http').get('http://localhost:3000/health', r => process.exit(r.statusCode === 200 ? 0 : 1))"
+
+CMD ["node", "server.js"]