Downloads the catppuccin-mocha theme from catppuccin/ghostty into
~/.config/ghostty/themes/catppuccin-mocha, which is the user theme
path Ghostty searches. Includes an idempotency check so re-running
bootstrap skips it if already installed.

Wire the script into debian/bootstrap and ubuntu/bootstrap inside
the desktop apps block, since ghostty is a desktop application.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>