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/Cargo.lock

@@ -921,6 +921,7 @@ dependencies = [
  "color-eyre",
  "crossterm",
  "diffy",
+ "dirs",
  "image",
  "insta",
  "itertools 0.14.0",