Fix "The sidebar of the repository file list does not have a fixed height #34298" (#34321)

There is a known issue where scrolling to the bottom of the page is
affected by unknown elements in the footer area:

https://github.com/go-gitea/gitea/blob/24145f811069295b9727f25469f1dd3a7c2c5dd7/templates/base/footer.tmpl#L11-L18

![after](https://github.com/user-attachments/assets/4cdbce32-d22e-4907-a78b-c8e301017fac)

web_src/css/repo/home.css

@@ -58,6 +58,11 @@
   flex: 0 0 15%;
   min-width: 0;
   max-height: 100vh;
+  position: sticky;
+  top: 0;
+  bottom: 0;
+  height: 100%;
+  overflow-y: hidden;
 }
 
 .repo-view-content {