feat(config): separate color palette configuration into colors.tomlFeat/separate colors config

[Silver-balls111]

Description

This PR separates the color palette configuration from config.toml into a dedicated colors.toml file (~/.config/ashell/colors.toml), while maintaining full backward compatibility with existing configurations.

Summary of Changes

• Separate colors.toml: Added support for reading palette colors from ~/.config/ashell/colors.toml (or --colors-path <path>).
• File watcher: Updated inotify subscription to watch and hot-reload changes from both config.toml and colors.toml.
• Backward Compatibility: If colors.toml does not exist, color fields configured in config.toml under [appearance] are automatically used as a fallback before defaulting to built-in colors.
• CLI Options: Added --colors-path argument to override the colors file location.
• Documentation: Updated mdBook reference docs and website documentation.

Testing & Quality

• make check passes (cargo fmt + cargo check + cargo clippy --all-features -- -D warnings).
• Unit tests added and passing for colors.toml parsing, backward compatibility fallback, and precedence.