remove blank scrollbars and use predefined components

[adityanandanx]

Description

This PR -

• replaces overflow-scroll with overflow-auto in the BrainsDropDown and in chats list.
• ChatsList, BrainsDropDown and ChatMessages now use the custom scrollbar style defined in the .scrollbar css class
• use predefined components in frontend/lib/components/ui, namely Field and Button in the BrainsDropDown component instead of HTML primitives
• add enter and exit animations using AnimatePresence for BrainsDropDown

Checklist before requesting a review

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented hard-to-understand areas
• I have ideally added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged

Screenshots:

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