Referrals: Add animated background gradient view to card.

[SergioEstevao]

| 📘 Part of: #2083 |
|:---:|

Fixes #

This PR implements the animated gradient view for the referrals card

Simulator.Screen.Recording.-.iPhone.15.Pro.-.2024-09-05.at.18.29.36.mp4

To test

1. Start the app
2. Ensure that you have the referrals FF enabled
3. Ensure that you have a Plus or Patron account
4. Open profile
5. Tap on the gift icon on the top left
6. Check that you see the animation like in the video above.
7. Please test on a real device to see performance

Checklist

• I have considered if this change warrants user-facing release notes and have added them to CHANGELOG.md if necessary.
• I have considered adding unit tests for my changes.
• I have updated (or requested that someone edit) the spreadsheet to reflect any new or changed analytics.

[dangermattic]

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[danielebogo]

It works as described! Looks good!

Just a nit: should we use Constants to define the values?

[SergioEstevao]

Just a nit: should we use Constants to define the values?

Are you suggesting using parameters for the color values, speed of animations and things like that?

I thought about it, but this animation is so specific to the Referrals card that I think it's not worth it to make it configurable.