feat: Add view stack to BottomPane (#4026)

Adds a "View Stack" to the bottom pane to allow for pushing/popping
bottom panels.

`esc` will go back instead of dismissing.

Benefit: We retain the "selection state" of a parent panel (e.g. the
review panel).

codex-rs/tui/src/app_event.rs

@@ -76,7 +76,4 @@ pub(crate) enum AppEvent {
 
     /// Open the custom prompt option from the review popup.
     OpenReviewCustomPrompt,
-
-    /// Open the top-level review presets popup.
-    OpenReviewPopup,
 }