ui: Design an "Add Custom Token" interface in the selection modal

[AlAfiz]

Description

Our default token list will only contain major verified assets. Users must be able to trade unlisted tokens by pasting a contract address.
When a user searches for a contract address that isn't in the default list, the UI needs to present a specific "Import Token" card.
This card should display a bold warning that the token is unverified and require them to check a box before adding it to their local list.
This allows permissionless trading while keeping TradeFlow legally protected from scam assets.

Requirements

• Update the Token Search modal logic: if the search input length is 56 (a standard Stellar address) and yields no default results, show the import UI.
• The import UI must display the truncated address and a red "Unknown Asset" warning badge.
• Add a checkbox labeled "I understand the risks of trading unverified tokens."
• Add an "Import" button that remains disabled until the checkbox is checked.