Support vim's j and k keybindings

I'm actually quite surprised if a TUI doesn't support j for down and k for up 😅

Look for the keybinding handling code in the TUI module, likely in a file like `src/UI.hs`. Identify where arrow key navigation is defined and add mappings for 'j' (down) and 'k' (up). Test the changes by running the application and verifying the new keybindings work in the PDF list view.

索引モデルが issue の本文から書いたものです。