feat: add document conversion support (Markdown, HTML, Plain Text)

- Add marked for Markdown to HTML conversion with GFM support
- Add turndown for HTML to Markdown conversion
- Add DOMPurify for HTML sanitization (security)
- Support Markdown ↔ HTML ↔ Plain Text conversions
- Add styled HTML output with responsive design
- Use client-side only DOMPurify to fix SSR issues

Supported conversions:
- Markdown → HTML (with code syntax, tables, blockquotes)
- HTML → Markdown (clean formatting preservation)
- Markdown/HTML → Plain Text (strip formatting)
- Plain Text → HTML/Markdown (basic formatting)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

README.md

@@ -7,7 +7,7 @@ A modern, browser-based file conversion application built with Next.js 16, Tailw
 - **🎬 Video Conversion** - Convert between MP4, WebM, AVI, MOV, MKV, and GIF
 - **🎵 Audio Conversion** - Convert between MP3, WAV, OGG, AAC, and FLAC
 - **🖼️ Image Conversion** - Convert between PNG, JPG, WebP, GIF, BMP, TIFF, and SVG
-- **📄 Document Conversion** - (Coming soon) Convert between PDF, DOCX, Markdown, HTML, and TXT
+- **📄 Document Conversion** - Convert between Markdown, HTML, and Plain Text
 - **🔒 Privacy First** - All conversions happen locally in your browser, no server uploads
 - **⚡ Fast & Efficient** - Powered by WebAssembly for near-native performance
 - **🎨 Beautiful UI** - Modern, responsive design with dark/light theme support
@@ -23,6 +23,9 @@ A modern, browser-based file conversion application built with Next.js 16, Tailw
 - **Tailwind CSS 4** - Utility-first CSS with OKLCH color system
 - **FFmpeg.wasm** - Video and audio conversion
 - **ImageMagick WASM** - Image processing and conversion
+- **Marked** - Markdown to HTML conversion
+- **Turndown** - HTML to Markdown conversion
+- **DOMPurify** - HTML sanitization
 - **Fuse.js** - Fuzzy search for format selection
 - **Lucide React** - Beautiful icon library
 
@@ -111,8 +114,14 @@ convert-ui/
 ### Images (ImageMagick)
 - **Input/Output:** PNG, JPG, WebP, GIF, BMP, TIFF, SVG
 
-### Documents (Coming Soon)
-- **Planned:** PDF, DOCX, Markdown, HTML, Plain Text
+### Documents
+- **Markdown → HTML** - Full GitHub Flavored Markdown support with styling
+- **HTML → Markdown** - Clean conversion with formatting preservation
+- **Markdown ↔ Plain Text** - Strip or add basic formatting
+- **HTML → Plain Text** - Extract text content
+- **Plain Text → HTML** - Convert to formatted HTML document
+
+**Note:** Uses lightweight JavaScript libraries (marked, turndown) instead of Pandoc WASM for fast, reliable conversions.
 
 ## How It Works