ui: Design a "Token Not Found" empty state for the search dropdown

[AlAfiz]

Description

When a user searches for a random string like "XYZ" in the token selection modal, the list simply goes blank.
A blank screen feels like a bug. We need a proper empty state that clearly indicates the token does not exist in our default list.
This empty state should also include a small hint telling them they can paste a custom contract address instead.
This massively improves the token discovery experience.

Requirements

• Update the TokenSelectModal.tsx component.
• If the filtered token array length is 0, render a centralized empty state div.
• Add an icon (like a magnifying glass) and the text: "No tokens found."
• Add secondary text: "Try pasting a valid Stellar contract address."