feat: add arrow key transcript scrolling

[SyahrulBhudiF]

Thanks for making this extension — I’ve been enjoying using it.

This PR adds a couple of extra ways to scroll the BTW overlay transcript:

• ArrowUp / ArrowDown scroll one line at a time
• mouse wheel / touchpad scrolling works in terminals that support SGR mouse events
• PgUp / PgDn still work as before for page scrolling
• the footer hint is updated so the scroll controls are easier to discover

The motivation is that not every keyboard has dedicated PgUp / PgDn keys, especially
on compact laptops/keyboards, so arrow keys and touchpad scrolling make the transcript
easier to navigate.

[dbachelder]

@SyahrulBhudiF thanks!