enhance(ui): add gap between file tree and diff on pr changes page (#37798)

### Before

<img width="410" height="324" alt="file-tree-before"
src="https://github.com/user-attachments/assets/cdc369a4-23b0-4f9c-8580-f87e84cb1400"
/>

### After

<img width="395" height="323" alt="file-tree-after"
src="https://github.com/user-attachments/assets/dd4932a6-f7f6-4b44-82cf-bcfaadadc18c"
/>

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

web_src/css/repo.css

@@ -1578,6 +1578,7 @@ tbody.commit-list {
 
 #diff-container {
   display: flex;
+  gap: var(--page-spacing);
 }
 
 #diff-file-boxes {