feat: scrolling down with Enter and up with alt+enter

[abhijeet-singh800]

Fixes #255

This PR adds a scroll-with-enter config option. When it's true, Enter scrolls the preview down one line and Alt+Enter scrolls it up one line (Default ; false). Preference given to the search when active.

Behavior Table

scroll-with-enter	Key	State	Result
off	Enter / Alt+Enter	preview	no change
off	Enter / Alt+Enter	active search	next match (unchanged)
on	Enter	preview	scroll down 1
on	Alt+Enter	preview	scroll up 1
on	Enter / Alt+Enter	active search	next match
on	Enter after Esc	preview	scroll down 1
any	Enter / Alt+Enter	code-select	copy block (unchanged)
any	Enter	search input	confirm query (unchanged)

• The new arms sit directly below the existing Enter if has_active_search() arm in keyboard.rs. The Alt arm comes before the bare Enter arm, otherwise Alt+Enter would scroll down.
• Code-select mode is unchanged: Enter still copies the block.
• Config only, no LEAF_* env var, same as watch and hyper-link-prefix.
• Strict bool, same as watch: an invalid value like "yes" falls back to defaults, as it already does for the other bool keys.
• Some terminals capture Alt+Enter (Windows Terminal toggles fullscreen), and macOS terminals need "Use Option as Meta". This is documented in config.toml. k / ↑ still work everywhere.

Testing

• cargo test
• cargo clippy --all-targets --all-features -- -D warnings, cargo build --release.
• Binary Test by Hand for Scroll Down with Enter and Scroll Up with Alt + Enter

Head Up : I built it with the help of AI and reviewed the changes and tested it by hand.