Request: Light mode / Dark mode support

[djensenius]

Hello!

I'd love to have Atuin support light mode / dark mode for the TUI.

If there is interest in support for this I could do the following (either non-AI assisted or non-vibe coded basic AI assistance, whichever is preferred by the community).

A rough example would be how bat does it:

name = "catppuccin-mocha-mauve"
dark = "catppuccin-mocha-mauve"
light = "catppuccin-latte-mauve"

To implement this I would suggest adding one additional crate: terminal-colorsaurus for OSC 11 terminal background detection then:

1. Add dark: Option and light: Option to ThemeDefinitionConfigBlock in crates/atuin-client/src/theme.rs
2. At theme load time, in ThemeManager::load_theme_from_config(), if dark/light are set, query the terminal color scheme and select the appropriate theme name
3. Fall back to name if detection fails or dark/light aren't configured

Thoughts?