[BUG] Undo functionality should support redo

Environment

• Platform (select one):
• [x] Anthropic API
• [ ] AWS Bedrock
• [ ] Google Vertex AI
• [ ] Other: <!-- specify -->
• Claude CLI version: 1.0.34 (Claude Code)
• Operating System: macos 15.3.1
• Terminal: ghostty

Bug Description

Right now ctrl+z does undo in the input box. I agree with https://github.com/anthropics/claude-code/issues/588 that ctrl+z should be respected to background the process, but separately — I have a habit of using ctrl+z to background applications, as is standard. That tic means that I am losing edits in the input box without the ability to ctrl+y or ctrl+r in vi mode to redo them.

Steps to Reproduce

1. undo
2. try to redo
3. it fails