Added forward/back selection during Esc-Esc backtrack preview (code ready)

What feature would you like to see?

Request

• Is it okay to open a PR for this? Code is ready, see commits in my fork:

https://github.com/fbal98/codex/tree/TUI-chat-history-backtrack-up-down-preview

Summary

• Add forward/back navigation while previewing backtrack targets in the transcript overlay.
• When preview (backtracking) is active:
• Up or Esc → move to the next older user message
• Down → move to the next newer user message
• Enter → confirm and fork (unchanged)
• Up/Down continue to have same behavior when preview is not active

Additional information

_No response_