feat(tui): Display keyboard shortcuts inline for approval options

Shows single-key shortcuts (y, a, n) next to approval options to make
them more discoverable. Previously these shortcuts worked but were
hidden, making the feature hard to discover.

Changes:
- "Yes, proceed" now shows "y" shortcut
- "Yes, and don't ask again" now shows "a" shortcut
- "No, and tell Codex..." continues to show "esc" shortcut

This improves UX by surfacing the quick keyboard shortcuts that were
already functional but undiscoverable in the UI.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: 0xRaduan

codex-rs/tui/src/bottom_pane/approval_overlay.rs

@@ -118,7 +118,8 @@ impl ApprovalOverlay {
             .iter()
             .map(|opt| SelectionItem {
                 name: opt.label.clone(),
-                display_shortcut: opt.display_shortcut,
+                display_shortcut: opt.display_shortcut
+                    .or_else(|| opt.additional_shortcuts.first().copied()),
                 dismiss_on_select: false,
                 ..Default::default()
             })