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>

lib/wasm/wasmLoader.ts

@@ -76,16 +76,30 @@ export async function loadImageMagick(): Promise<any> {
 }
 
 /**
- * Load Pandoc WASM module (placeholder for future implementation)
+ * Load Pandoc converter (uses pure JavaScript libraries, not WASM)
+ * Note: We use marked + turndown instead of actual Pandoc WASM
  */
 export async function loadPandoc(): Promise<any> {
   if (pandocInstance && moduleState.pandoc) {
     return pandocInstance;
   }
 
-  // TODO: Implement Pandoc WASM loading when available
-  // For now, throw an error
-  throw new Error('Pandoc WASM module is not yet implemented');
+  try {
+    // Import the converter libraries
+    const [marked, turndown] = await Promise.all([
+      import('marked'),
+      import('turndown'),
+    ]);
+
+    pandocInstance = { marked, turndown };
+    moduleState.pandoc = true;
+    console.log('Document converter loaded successfully');
+
+    return pandocInstance;
+  } catch (error) {
+    console.error('Failed to load document converter:', error);
+    throw new Error('Failed to load document converter');
+  }
 }
 
 /**