ux: Refactor Token Select Modal into a Bottom Sheet for mobile

[AlAfiz]

Description

On desktop, a centered popup modal for selecting tokens looks great. On a mobile phone, it feels cramped and hard to use.
Modern mobile applications use a "Bottom Sheet" pattern (sliding up from the bottom of the screen) for selection menus.
We need to adjust the CSS of our token selection modal so that on small screens (sm breakpoint), it snaps to the bottom edge.
This will make TradeFlow feel like a premium native iOS/Android app when accessed via a mobile wallet browser.

Requirements

• Open the TokenSelectModal.tsx component.
• Adjust the wrapper div classes using Tailwind responsive prefixes.
• On sm: and above, keep it centered (inset-0 flex items-center justify-center).
• On mobile (default), align it to the bottom (fixed bottom-0 left-0 right-0 rounded-t-2xl pb-safe).