feat: support mainnet in addition to testnet

[Hollujay]

Summary

The tool targets testnet. Anyone wanting to inspect a real production contract
before interacting with it — arguably the higher-value case, since that's where
mistakes cost money — has no path.

Acceptance Criteria

• Add a network selector (testnet / mainnet) in the UI
• Network choice drives the RPC endpoint and network passphrase together,
  so they can never mismatch
• Persist the selection across reloads
• Make the active network unmistakable in the UI — a user must never think
  they're simulating against mainnet when they're on testnet, or vice versa
• Document the mainnet RPC default in .env.example

Tech Stack

TypeScript, React