Enhancement: watermark feature

Currently, the watermark feature tiles the image across the page, appearing multiple times. Could we have a customisable watermark feature e.g., positioning, opacity, placing a single image or text rather than tiled?

The issue does not name files, tests, or an entry point. Locate the existing watermark implementation and review how it tiles images, then clarify the supported positioning, opacity, and image-versus-text behavior before estimating the work. Done should mean the requested watermark options work without unwanted tiling.

Written by the indexing model from the issue text.