Provide PDF preview before download

[jeisaacs]

Summary

Offer an inline PDF preview so users can verify ordering/rotation before saving the merged document.

Why

• Prevents downloading incorrect mixes when files were swapped or mis-ordered
• Improves trust when running on mobile devices without native PDF tooling
• Enables quick spot checks of specific pages

Proposal

• Generate a preview blob after processing and render it via a PDF viewer component
• Allow toggling between thumbnail grid and full-page view
• Ensure previews respect dark/light theme and remain performant for large files

Acceptance Criteria

• Preview is available immediately after processing without extra clicks
• Users can move between pages and zoom without leaving the app
• Download button is still provided for final export
• Feature gracefully degrades for extremely large PDFs or unsupported browsers