feat: PDF export option

[ikorich]

Summary

Add ability to export comparisons as PDF files.

Motivation

PDF format is preferred for documentation and printing.

Implementation

• Add PDF option to export
• Use PDFKit or ImageRenderer with PDF context
• Maintain vector quality where possible

Files to Modify

• ContentView.swift - PDF export logic
• ToolBarView.swift - Export format selection

Acceptance Criteria

• PDF export option available
• PDF renders correctly
• Text remains selectable in PDF
• File saves to selected location

Story Points

3 points