feat: update splash (#3631)

- Update splash styling.
- Add center truncation for long paths.
  (Uses new `center_truncate_path` utility.)
- Update the suggested commands.


## New splash
<img width="560" height="326" alt="image"
src="https://github.com/user-attachments/assets/b80d7075-f376-4019-a464-b96a78b0676d"
/>

## Example with truncation:
<img width="524" height="317" alt="image"
src="https://github.com/user-attachments/assets/b023c5cc-0bf0-4d21-9b98-bfea85546eda"
/>

codex-rs/tui/src/chatwidget/tests.rs

@@ -781,8 +781,7 @@ async fn binary_size_transcript_snapshot() {
     // Consider content only after the last session banner marker. Skip the transient
     // 'thinking' header if present, and start from the first non-empty content line
     // that follows. This keeps the snapshot stable across sessions.
-    const MARKER_PREFIX: &str =
-        "Describe a task to get started or try one of the following commands:";
+    const MARKER_PREFIX: &str = "To get started, describe a task or try one of these commands:";
     let last_marker_line_idx = lines
         .iter()
         .rposition(|l| l.trim_start().starts_with(MARKER_PREFIX))