Implement Select Asset and Select Network Screens [Flutter] #50
Description
Build two standalone selection screens used across the app. The "Select asset" screen lists available assets (USDC, USDT) with icon, full name, USD balance, and token balance per row. The "Select network" screen lists supported networks (Stellar) with icon, name, USD balance, and token balance. Both screens have a back navigation arrow and a title.
Acceptance Criteria:
- Select Asset screen renders all supported assets with icon, name, and balances
- Select Network screen renders all supported networks with icon, name, and balances
- Tapping an item selects it, closes the screen, and returns the selection to the caller
- Both screens support back navigation via the arrow
- Zero balances display as "$0.00 / 0 USDC"
- Screens are reusable and accept a callback or route parameter for the selected value