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/chatwidget/snapshots/codex_tui__chatwidget__tests__apply_patch_manual_flow_history_approved.snap

@@ -3,4 +3,4 @@ source: tui/src/chatwidget/tests.rs
 expression: lines_to_single_string(&approved_lines)
 ---
 • Added foo.txt (+1 -0)
-    1     +hello
+    1 +hello

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__approval_modal_patch.snap

@@ -4,12 +4,12 @@ expression: terminal.backend().vt100().screen().contents()
 ---
   Would you like to make the following edits?
 
+  Reason: The model wants to apply changes
+
   README.md (+2 -0)
 
-      1     +hello
-      2     +world
-
-  The model wants to apply changes
+    1 +hello
+    2 +world
 
 › 1. Yes, proceed
   2. No, and tell Codex what to do differently esc