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>

.gitignore

@@ -0,0 +1,6 @@
+node_modules/
+*.html
+*.pdf
+!server.js
+!convert.js
+!md2html.js