feat: add compose pay button and processing

[saralvasquez]

Summary of changes

• adds pay button
• adds backend processing like card client init and submit on pay button click
• resolves a UI bug for the hint float height after recomposition

Screen_recording_20260819_115504.webm

AI Usage

Which AI Agent Was Used?

• Copilot
• Claude
• Other (Type Name Here)

How was AI used?
Used the find cause of UI bug, wire back end processing, and write unit tests

Estimated AI Code Contribution

• less than 30%
• 30 - 60%
• 60 - 100%

Checklist

• Added a changelog entry
• Tested and confirmed payment flows affected by this change are functioning as expected

Authors

• @saralvasquez

[noguier]

Just a couple of non-blocking comments. Everything looks and functions great :)

[buzzamus]

These comments are very helpful!

[saralvasquez]

Okie dokie! Take a look at the latest commit. I amended it so there's a lot in there but I made some changes to how rememberCardFieldsController works. It mostly functions as a constructor this way. I was going to rename it to CardFieldsController but apparently the remember keyword is a compose standard naming convention to signal that it's remembered on the back end so the robot suggested I don't change it