fix: preserve scroll position when locking body

[wanxiankai]

Summary

• preserve the current window scroll position when useLockBodyScroll mounts
• lock the body with fixed positioning while keeping the viewport at the same visual offset
• restore the previous inline body styles and scroll position on cleanup

Fixes #342.

Verification

• node --check index.js
• node -e "import('./index.js').then((m) => { if (typeof m.useLockBodyScroll !== 'function') process.exit(1) })"
• temporary React dispatcher smoke test for useLockBodyScroll style restoration and scrollTo behavior

Note: npx tsc --noEmit currently fails on existing site TypeScript configuration/dependencies unrelated to this change (missing JSX config and site packages).