Fix: Add 'stable' css styling to scrollbar-gutter to fix layout shifts when opening modals

[ShpetimA]

https://github.com/sst/opencode/issues/5848

To fix this I added scrollbar-gutter: "stable" styling to avoid layout shifting when opening modals like cmd+k or other things that cause the scrollbar to disappear. https://developer.mozilla.org/en-US/docs/Web/CSS/scrollbar-gutter

Without srollbar-gutter layout shifts:

https://github.com/user-attachments/assets/e07242c6-7002-4a2d-b73d-f671bfabc4be

With scrollbar-gutter: "stable" no layout shift:

https://github.com/user-attachments/assets/afbf62ce-906d-4bcb-8f80-4ad85fa32f9d