Align user history message prefix width (#3467)

<img width="798" height="340" alt="image"
src="https://github.com/user-attachments/assets/fdd63f40-9c94-4e3a-bce5-2d2f333a384f"
/>

codex-rs/tui/src/ui_consts.rs

@@ -0,0 +1,10 @@
+//! Shared UI constants for layout and alignment within the TUI.
+
+/// Width (in terminal columns) reserved for the left gutter/prefix used by
+/// live cells and aligned widgets.
+///
+/// Semantics:
+/// - Chat composer reserves this many columns for the left border + padding.
+/// - Status indicator lines begin with this many spaces for alignment.
+/// - User history lines account for this many columns (e.g., "▌ ") when wrapping.
+pub(crate) const LIVE_PREFIX_COLS: u16 = 2;