fix(ui-modal): adjust scrollbar detection tolerance in ModalBody

Increase epsilon tolerance from 0.05 to 1 pixel to avoid false positives
when detecting scrollbars due to browser rounding errors. This prevents
unnecessary tabIndex=0 from being applied when no scrollbar is present.

Fixes INSTUI-4863

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: matyasf

packages/ui-modal/src/Modal/ModalBody/index.tsx

@@ -130,12 +130,12 @@ class ModalBody extends Component<ModalBodyProps> {
         css={this.props.styles?.modalBody}
         padding={padding}
         // check if there is a scrollbar, if so, the element has to be tabbable to be able to scroll with keyboard only
-        // epsilon tolerance is used to avoid false positives, this is generally safer than Math rounding techniques
+        // epsilon tolerance is used to avoid scrollbar for rounding errors
         {...(finalRef &&
         Math.abs(
           (finalRef.scrollHeight ?? 0) -
             (finalRef.getBoundingClientRect()?.height ?? 0)
-        ) > 0.05
+        ) > 1
           ? { tabIndex: 0 }
           : {})}
       >