Add configurable list selection styling

[daewon]

Closes #95.

Summary

• add [ui.theme] settings for selected row colors
• add optional object_dir_bold
• preserve the existing default styling when the config is unset

Validation

• cargo fmt --all -- --check
• cargo test -q
• cargo clippy --all-targets --all-features -- -D warnings

[lusingander]

Thank you for your pull request.
However, it seems we need to discuss the implementation policy.

• I believe that what is currently defined in ColorTheme should be configurable.
  • I think it would be good to directly deserialize into the ColorTheme struct.
  • Ratatui's serde feature provides deserialization for Color.
    • Example from my other project:
      • https://github.com/lusingander/cargo-selector/blob/70cd637f3f92a00b4182c9fa6fcef90e803a9cb0/src/config.rs#L33
      • https://github.com/lusingander/serie/blob/cd6c194f3f0acb3f93ba7ff2d93abbde30034bc5/src/color.rs#L10
• How about renaming ColorTheme to Theme and adding bold settings as well?

If you're interested in making the changes, I'd be happy if you could consider it.
Even if you're not interested, I'd like to implement this feature sometime soon if I get around to it.

[daewon]

@lusingander Thank you for the feedback.

I really enjoy using STU, and I think it is a wonderful applicatiton with a great TUI design.

I’ll try implementing it in the direction you suggested and update the PR.

[daewon]

@lusingander I updated the implementation in the direction you suggested.

• Renamed ColorTheme to Theme
• Deserialized [ui.theme] directly into Theme
• Switched to Ratatui's Color serde support and removed the manual color parsing layer
• Made the existing theme fields configurable
• Moved object_dir_bold into Theme
• Updated the docs and tests accordingly

I also simplified the structure a bit so the theme is now read directly from the config as the single source of truth.

If there are still points that should be revised, please feel free to let me know.