[Swap] Default the "You receive" token to USDC

[CassioMG]

Summary

In the Swap flow, the You receive field currently defaults to an empty state with a Select button (and a + icon on the left). We should default the receive token to USDC instead, so users have one less tap to complete a common swap.

Current behavior

• The You receive field shows a Select button (empty state).
• A + icon is shown on the left of the field.

Desired behavior

• Default the You receive token to USDC, except when starting the swap flow from the USDC Asset Details screen in which case the You receive token should default to the native XLM.
• Apply this default regardless of whether the user already holds a USDC balance.
• Users can still open the selector to choose a different receive token.
• If we keep an empty/select state anywhere, remove the + icon on the left (see thread note)

Source

Based on Slack discussion: https://stellarfoundation.slack.com/archives/C03DK78B43C/p1783960898557799