Add `gg`/`G` (vim), `cmd-up`/`cmd-down` (macOS), and
`ctrl-home`/`ctrl-end` (Linux/Windows) keybindings to scroll to the top
and bottom of the markdown preview.

The markdown preview already has page scroll (`ctrl-d`/`ctrl-u`), line
scroll (`ctrl-e`/`ctrl-y`), and item scroll (`alt-up`/`alt-down`) but
was missing top/bottom navigation. This adds two new actions —
`ScrollToTop` and `ScrollToBottom` — using the existing
`ListState::scroll_to()` infrastructure, following the same pattern as
the other scroll actions.

- [x] Done a self-review taking into account security and performance
aspects

Release Notes:

- Added scroll-to-top and scroll-to-bottom keybindings for markdown
preview (`gg`/`G` in vim mode, `cmd-up`/`cmd-down` on macOS,
`ctrl-home`/`ctrl-end` on Linux/Windows)