fix: preserve scroll position in multi-select on option toggle

[govindarajug]

Summary

When selecting or deselecting options in a multi-select dropdown, the menu list jumps back to the top. This is a long-standing issue that makes multi-select unusable when working with large option lists.

Root cause: The useEffect in MenuList that handles scroll behavior unconditionally calls list.current.scrollToItem(currentIndex) whenever children changes. In multi-select mode, every option toggle triggers a children re-render, resetting the scroll to the focused item (often index 0).

Fix:

• Track the current scroll offset via a ref, updated through react-window's onScroll callback
• On re-render in multi-select mode, restore the saved scroll position instead of scrolling to currentIndex
• Single-select keyboard navigation behavior is preserved unchanged

Changes

• Added scrollOffsetRef to track scroll position via List's onScroll prop
• Modified the scroll restoration useEffect to check isMulti — if true and a scroll offset exists, restore it; otherwise fall back to the existing scrollToItem behavior

Test plan

• TypeScript compiles cleanly (tsc --noEmit --skipLibCheck)
• Open a multi-select with 100+ options, scroll down, select an option — menu should stay at the same scroll position
• Open a multi-select, scroll down, deselect an option — menu should stay at the same scroll position
• Open a single-select, use keyboard arrows to navigate — should scroll to focused item as before
• Open a multi-select with grouped options — scroll persistence should work the same

Fixes #72, #106, #142