feat: add share feature
UI: minor updates

This update refines how MarkD appears on the App Store:

• Clearer description and improved search keywords
• More accurate category — now classified under Developer Tools
• Same focused reading experience for Markdown, code, and Mermaid diagrams

• Document Sharing: Share Markdown, HTML, and code files to other apps.
• Faster Loading: Documents now open ~40% faster.
• File Name Display: Original file names are shown as titles in the list and detail views.