feat(themes): support "none" color for the terminal default

[ChrisJr404]

📺 PR Description

This adds a "none" pseudo-color that maps to the terminal's default color, so a theme (or a theme_overrides entry) can leave an element uncolored instead of forcing a concrete color. It closes #1103.

The motivation is transparency: when you start from a builtin theme but want the background to fall through to your terminal emulator (which may be transparent), there was previously no way to express that through an override — every value had to be a concrete ANSI or hex color. background = "none" now keeps the terminal's own background. The same works for any foreground field if you'd rather inherit the terminal default there too.

Under the hood this is a new Color::Reset variant that resolves to ratatui's Color::Reset. Parsing goes through the single Color::from_str used by both theme files and overrides, so both paths pick it up. "none" is the primary spelling; "default" and "reset" are accepted as aliases. Existing themes are unaffected — no builtin uses these words.

Docs: added a line to the theme color reference in docs/user-guide/05-themes.md.

Checklist

• my commits and PR title follow the conventional commits format
• if this is a new feature, I have added tests to consolidate the feature and prevent regressions
• if this is a bug fix, I have added a test that reproduces the bug (if applicable)
• I have added a reasonable amount of documentation to the code where appropriate

Tests

Added four unit tests in config::themes: "none"/"default"/"reset" (case-insensitive) parse to Color::Reset; Color::Reset converts to ratatui::style::Color::Reset; background = "none" deserializes from a theme file; and an override of background = "none" merges correctly. cargo test --lib themes passes (11/11), and cargo fmt --check / cargo clippy --lib are clean.