TUI: allow stepping forward during backtrack message edit

When using the TUI backtrack/edit flow (press Esc then Esc to edit a previous user message), Esc currently steps to older messages only. If you overshoot the intended message while paging backward, there is no way to move forward to the next (newer) message.

Proposal:

• When backtrack preview is active (a user message is highlighted), support ← (older) and → (newer).
• Keep the existing Esc behavior (enter preview and step to older messages).
• No wrap-around; clamp at oldest/newest.
• Update the transcript overlay footer hints to reflect the new navigation.