Scrollable slash commands (#1830)

Scrollable slash commands. Part 1 of the multi PR.

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

@@ -0,0 +1,5 @@
+//! Shared popup-related constants for bottom pane widgets.
+
+/// Maximum number of rows any popup should attempt to display.
+/// Keep this consistent across all popups for a uniform feel.
+pub(crate) const MAX_POPUP_ROWS: usize = 8;