fix: faster message restore and skip panel animation on refresh

[DayuanJiang]

Problem

• Chat messages appeared after the panel on page refresh (noticeable delay)
• Panel slide-in animation played on every page refresh, causing unwanted visual shift

Solution

• Use useLayoutEffect instead of useEffect for localStorage restore - runs synchronously before browser paint
• Track visibility state changes to only animate panel when user toggles it, not on page load
• Use cn() utility for cleaner conditional className