Feature-gate undo module behind input-components

The undo module is only used by InputField and TextArea (both behind
the input-components feature), but was always compiled. This produced
dead-code warnings under --no-default-features. Gate the module
declaration with #[cfg(feature = "input-components")] to eliminate
those warnings.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ryanoneill

src/lib.rs

@@ -108,6 +108,7 @@ pub mod layout;
 pub mod overlay;
 pub mod style;
 pub mod theme;
+#[cfg(feature = "input-components")]
 pub(crate) mod undo;
 
 // Re-export commonly used types