Add configuration option for PDF preview method (text vs image)

Problem

The current PDF preview implementation converts PDFs to images using pdftoppm, which can be slow for large PDFs or when browsing through multiple PDF files.

Proposed Solution

Add a user configuration option to choose between two PDF preview methods:

• Text-based preview: Use pdftotext to extract and display text content (faster, but no visual preview)
• Image-based preview: Use pdftoppm to convert the first page to an image thumbnail (slower, but shows visual layout)

This would allow users to balance between performance and visual fidelity based on their needs.

Context

• Introduced in PR #1198
• Requested by @lazysegtree in comment