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>

package.json

@@ -13,18 +13,22 @@
     "@ffmpeg/util": "^0.12.1",
     "@imagemagick/magick-wasm": "^0.0.30",
     "clsx": "^2.1.1",
+    "dompurify": "^3.2.2",
     "fuse.js": "^7.1.0",
     "lucide-react": "^0.553.0",
+    "marked": "^15.0.4",
     "next": "^16.0.0",
     "react": "^19.0.0",
     "react-dom": "^19.0.0",
-    "tailwind-merge": "^3.3.1"
+    "tailwind-merge": "^3.3.1",
+    "turndown": "^7.2.0"
   },
   "devDependencies": {
     "@tailwindcss/postcss": "^4.1.17",
     "@types/node": "^22",
     "@types/react": "^19",
     "@types/react-dom": "^19",
+    "@types/turndown": "^5.0.5",
     "eslint": "^9",
     "eslint-config-next": "^16.0.0",
     "tailwindcss": "^4.0.0",