TUI: Ctrl/Cmd+Z should restore draft cleared by Ctrl+C

In the TUI chat composer, Ctrl+C clears the draft (and records the cleared draft into local composer history so it can be recalled via Up).

Suggestion: when the composer is empty, treat Ctrl+Z (and Super/Cmd+Z where available) as an "undo clear" shortcut that restores the most recently-cleared local draft.

Why

• Matches common user expectation: undo after an accidental clear.
• Reuses existing history rehydration logic; no need for full undo/redo.

Proposed behavior

• Only when composer is empty (so it does not act as a general undo system).
• Key chord: Ctrl+Z and Super+Z (ignore Alt-modified).
• Action: history.navigate_up(...) + apply_history_entry(...).

Implementation sketch

• codex-rs/tui/src/bottom_pane/chat_composer.rs: in handle_key_event_without_popup, add a match arm for KeyCode::Char('z') with CONTROL or SUPER modifiers and self.is_empty().
• Add a small unit test that clears a draft via clear_for_ctrl_c() then sends Ctrl+Z and asserts the draft text is restored.

Note: this is additive; Up-arrow history recall continues to work as today.