Integrate Coinbase onramp

[piyalbasu]

Given the findings from Bruno and Piyal
https://docs.google.com/document/d/1PATc6zh5RvwQ3SDDYN462GeqmEUUulkB9feNBaPYb9Q/edit?tab=t.0#heading=h.i8g2bve1not7
https://docs.google.com/document/d/1cHYQRocIgV_RFcYD0cipa2Q3glAFfwG1y9Dt1ovM6uc/edit?tab=t.0

We appears to be leaning in the direction of integrating with Coinbase as an on ramp. Coinbase provides a URL-based solution that opens in a new tab.

Acceptance Criteria:

In Freighter, implement a button that links out to this experience and sets reasonable defaults like:

1. destination account
2. asset type

Use this designs to stylize the button that leads the user to this experience:
https://www.figma.com/design/C3G0a4Gd6RQyplRBppGDsL/Freighter-(SDS-v3)?node-id=1127-5129&t=PJprU2dPFHyPfAPB-0

NOTE: this design specifies Moonpay. Once confirmed, work with Charles to replace with a Coinbase button