Summary

• Add .md -> text/markdown mapping to getContentType() in src/utils/url.js so .md files are served with the correct MIME type
• Add text/markdown to the rdfTypes array in shouldServeMashlib() in src/mashlib/index.js so Mashlib renders markdown resources when accessed via browser

Fixes #226

Test plan

• Verify .md files are served with Content-Type: text/markdown
• Verify Mashlib intercepts and renders .md resources when browsing with Accept: text/html
• Verify existing content type mappings and mashlib behavior are unchanged