This PR implements the dynamic network switching capability as requested in issue #80.

[Ahavahdev1]

Key Changes

• Centralized network configurations into a structured NETWORKS object, supporting both testnet and mainnet.
• Implemented getSelectedNetwork using localStorage with SSR safety checks.
• Refactored SoroSaveClient initialization to dynamically use the active network configuration.
• Exported network constants for consistent usage across the application.

Closes #80
@Opire solve