ux: Implement a specialized error page for unsupported networks

[AlAfiz]

Description

If a user connects their Freighter wallet but they are on the Stellar "Public" network while our app is on "Testnet", all transactions will fail.
Instead of showing generic errors, we need a dedicated full-screen overlay that blocks the UI and tells them to switch networks.
This is a critical UX pattern that prevents users from getting frustrated with failed contract calls.
It ensures the app environment and wallet environment are always perfectly synced.

Requirements

• Read the current network string from the connected wallet state.
• Compare it against the expected network defined in the frontend environment variables.
• If they do not match, render a full-screen UnsupportedNetwork.tsx component.
• The component must feature a bold warning and a clear instruction to "Switch your wallet to Testnet".