refactor: remove all document conversion support, keep only media conversions

This commit completely removes document conversion functionality to focus
exclusively on media file conversions (video, audio, images).

Changes:
- Remove all document converter services (pandocService.ts, pdfService.ts, docxService.ts)
- Uninstall document-related packages: marked, turndown, dompurify, jspdf, pdfjs-dist, docx, mammoth, @types/turndown
- Remove document formats (PDF, DOCX, Markdown, HTML, TXT) from formatMappings.ts
- Remove pandoc converter from FileConverter.tsx
- Remove pandoc loader and references from wasmLoader.ts
- Update TypeScript types to remove 'pandoc' from ConverterEngine and 'document' from FileCategory
- Remove pandoc from WASMModuleState interface
- Update README.md to remove all document conversion documentation
- Update UI descriptions to reflect media-only conversions

Supported conversions now:
- Video: MP4, WebM, AVI, MOV, MKV, GIF
- Audio: MP3, WAV, OGG, AAC, FLAC
- Images: PNG, JPG, WebP, GIF, BMP, TIFF, SVG

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

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

lib/utils/formatMappings.ts

@@ -162,53 +162,6 @@ export const SUPPORTED_FORMATS: ConversionFormat[] = [
     converter: 'imagemagick',
     description: 'Scalable Vector Graphics',
   },
-
-  // Document formats (Pandoc - future implementation)
-  {
-    id: 'pdf',
-    name: 'PDF',
-    extension: 'pdf',
-    mimeType: 'application/pdf',
-    category: 'document',
-    converter: 'pandoc',
-    description: 'Portable Document Format',
-  },
-  {
-    id: 'docx',
-    name: 'DOCX',
-    extension: 'docx',
-    mimeType: 'application/vnd.openxmlformats-officedocument.wordprocessingml.document',
-    category: 'document',
-    converter: 'pandoc',
-    description: 'Microsoft Word document',
-  },
-  {
-    id: 'markdown',
-    name: 'Markdown',
-    extension: 'md',
-    mimeType: 'text/markdown',
-    category: 'document',
-    converter: 'pandoc',
-    description: 'Markdown text',
-  },
-  {
-    id: 'html',
-    name: 'HTML',
-    extension: 'html',
-    mimeType: 'text/html',
-    category: 'document',
-    converter: 'pandoc',
-    description: 'HyperText Markup Language',
-  },
-  {
-    id: 'txt',
-    name: 'Plain Text',
-    extension: 'txt',
-    mimeType: 'text/plain',
-    category: 'document',
-    converter: 'pandoc',
-    description: 'Plain text file',
-  },
 ];
 
 /**