Feature: Add support for PDF documents in expenses (#397)

[a-camacho]

This PR implements support for uploading and viewing PDF documents alongside images for expense records, addressing the feature request in #397.

Changes included:

• Database Schema: Updated ExpenseDocument model in prisma/schema.prisma to make width and height optional, as these dimensions are not applicable to PDF files.
• Validation: Adjusted expenseFormSchema in src/lib/schemas.ts to allow nullish values for document dimensions.
• UI - Upload: Modified ExpenseDocumentsInput to accept application/pdf. The upload logic now conditionally fetches image dimensions only if the file type is an image.
• UI - Preview:
  • Added a fallback FileText icon for PDF thumbnails in the document list.
  • Implemented an iframe viewer within the document carousel to allow users to view PDF content directly within the application.
• Migration: Included a migration script to drop NOT NULL constraints on existing document dimension columns.

Disclaimer : I used AI tools to help draft this implementation; however, I have personally audited and tested all code changes—including the database migration and UI logic—to ensure they are correct and functional.