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>

types/conversion.ts

@@ -1,12 +1,12 @@
 /**
  * Supported converter engines
  */
-export type ConverterEngine = 'ffmpeg' | 'imagemagick' | 'pandoc';
+export type ConverterEngine = 'ffmpeg' | 'imagemagick';
 
 /**
  * File category based on type
  */
-export type FileCategory = 'video' | 'audio' | 'image' | 'document';
+export type FileCategory = 'video' | 'audio' | 'image';
 
 /**
  * Conversion status
@@ -65,11 +65,6 @@ export interface ConversionOptions {
   imageHeight?: number;
   imageFormat?: string;
 
-  // Document options
-  documentPageSize?: string;
-  documentMargins?: string;
-  documentStyles?: boolean;
-
   // Generic options
   [key: string]: string | number | boolean | undefined;
 }
@@ -108,7 +103,6 @@ export interface FormatPreset {
 export interface WASMModuleState {
   ffmpeg: boolean;
   imagemagick: boolean;
-  pandoc: boolean;
 }
 
 /**