dynamic width for line numbers in diffs (#4664)

instead of always reserving 6 spaces for the line number and gutter, we
now dynamically adjust to the width of the longest number.

<img width="871" height="616" alt="Screenshot 2025-10-03 at 8 21 00 AM"
src="https://github.com/user-attachments/assets/5f18eae6-7c85-48fc-9a41-31978ae71a62"
/>
<img width="871" height="616" alt="Screenshot 2025-10-03 at 8 21 21 AM"
src="https://github.com/user-attachments/assets/9009297d-7690-42b9-ae42-9566b3fea86c"
/>
<img width="871" height="616" alt="Screenshot 2025-10-03 at 8 21 57 AM"
src="https://github.com/user-attachments/assets/669096fd-dddc-407e-bae8-d0c6626fa0bc"
/>

codex-rs/tui/src/render/mod.rs

@@ -4,6 +4,7 @@ pub mod highlight;
 pub mod line_utils;
 pub mod renderable;
 
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub struct Insets {
     pub left: u16,
     pub top: u16,