feat: Add a "Transaction Deadline" input to advanced swap settings

[AlAfiz]

Description

If the Stellar network becomes heavily congested, a user's trade might sit pending for minutes.
If the price shifts dramatically during that time, the trade could execute at a terrible rate.
We need to add a "Transaction Deadline" (e.g., 20 minutes) to the settings modal.
If the transaction hasn't confirmed by this deadline, the smart contract will automatically reject it.

Requirements

• Add a new number input field to the existing Settings Modal labeled "Transaction Deadline (minutes)".
• Set the default value to 20 minutes.
• Save this value to the global React state or context alongside the slippage tolerance.
• Display a small "Deadline: 20m" indicator inside the main swap confirmation card.