Add optional OP_RETURN field to Send screen

[EricGrill]

Summary

• Adds an optional OP_RETURN field to each payment tab on the Send screen
• Allows users to embed arbitrary data (up to 80 bytes) in their Bitcoin transactions
• The OP_RETURN output has 0 value as required by the Bitcoin protocol

Implementation Details

• New OP_RETURN text field added to the payment form (payment.fxml)
• Real-time byte count display with validation status indicator
• Validation for the 80-byte maximum limit with visual feedback
• OP_RETURN data is encoded as UTF-8
• Multiple payments can each have their own OP_RETURN data
• Existing OP_RETURN functionality (BIP47 notifications) is preserved and works alongside user-entered data

Technical Changes

• PaymentController.java: Added FXML field declarations, getOpReturnData(), isValidOpReturn(), updateOpReturnStatus(), and clearOpReturn() methods
• SendController.java: Added getOpReturnsFromPaymentTabs() and areAllOpReturnsValid() methods, modified updateTransaction() to combine user-entered OP_RETURN data with existing opReturnsList
• payment.fxml: Added new OP_RETURN field with tooltip and status indicator

Screenshots

The OP_RETURN field appears below the Amount field in each payment tab, with a status indicator showing the byte count and validation state.

Test plan

• Verify the OP_RETURN field appears on the Send screen
• Test entering data and observing the byte count
• Test exceeding 80 bytes and verify validation error
• Create a transaction with OP_RETURN data and verify it's included in the transaction
• Test clearing the form and verify OP_RETURN field is cleared
• Test multiple payment tabs with different OP_RETURN data
• Verify BIP47 notification transactions still work correctly

Closes #97

🤖 Generated with Claude Code

[tigernero79]

Can I know in which version this very useful feature will be implemented, at least for me?

Or how can I use it today with Sparrow Wallet 2.4.1?