Add PDF Preview for "Find Unlinked Files"

JabRef offers [a Wizard to link non-linked files](https://docs.jabref.org/collect/findunlinkedfiles#using-the-wizard-search-for-unlinked-local-files) When clicking on a PDF, one has no clue, what JabRef will do <img width="1145" height="876" alt="Image" src="https://github.com/user-attachments/assets/456b0e6e-df61-4d16-ab2c-38c0a8f04a9d" /> it would be nice to have a second column next to the tree, which displays - the PDF (using JabRef's internal PDF viewer) - extracted meta data One can deactive the preview so that JabRef doesn't use the respective servcies at each click. -- maybe with collapsing and uncollapsing?