Feature-gate undo module behind input-components

[ryanoneill]

Summary

• Gate pub(crate) mod undo with #[cfg(feature = "input-components")] in src/lib.rs
• The undo module is only used by InputField and TextArea (both behind input-components), but was always compiled, producing dead-code warnings under --no-default-features

Verification

• cargo check --no-default-features produces zero warnings
• cargo check --all-features produces zero warnings
• All undo-related tests pass

Test plan

• cargo check --no-default-features — zero warnings
• cargo test --all-features -- undo — all 67 tests pass

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.61%. Comparing base (087bee6) to head (9a3de2c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #113   +/-   ##
=======================================
  Coverage   92.61%   92.61%           
=======================================
  Files         100      100           
  Lines        9530     9530           
=======================================
  Hits         8826     8826           
  Misses        704      704           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.