[drawer] Scroll lock applies html `scrollbar-gutter: stable`

# Bug report

## Current behavior

- `html.style.scrollbarGutter = "stable"` is applied while the drawer is open
- A visible gutter appears outside the drawer
- The gutter belongs to the page root, not the drawer

## Expected behavior

- no visible gutter outside the drawer
- gutter compensation should not be visible for side drawers
- a prop/config should exist to disable this behavior
-
## Reproducible example

1. Open https://base-ui.com/react/components/drawer
2. Open the right-side drawer example
3. Inspect `html`
4. Observe inline style: `scrollbar-gutter: stable`
5. Notice the reserved gutter/blank strip outside the drawer

## Base UI version

^1.3.0

## Which browser are you using?

Chrome

## Which OS are you using?

Ubuntu & Windows

## Which assistive tech are you using (if applicable)?

Voiceover/JAWS etc.

## Additional context

This may be intentional for layout shift prevention, but for side drawers the reserved root gutter becomes visibly detached from the drawer and looks broken