feat: WalletConnect v2 connect button template

[satyakwok]

Add a minimal WalletConnect v2 wallet-connect button template so dApp builders see how to wire it up cleanly on Sentrix.

Why

WalletConnect is the de facto wallet-abstraction layer; most users-of-Sentrix dApps will come in via WalletConnect-compatible wallets (Solux, MetaMask via WC, Rainbow, etc.). The starter should show the canonical way.

Scope

• React + viem + @walletconnect/web3wallet minimal setup
• ConnectButton component
• Sign + send example tx via the connected wallet
• Network-switch handler (auto-prompt to add Sentrix Mainnet / Testnet)

Cross-reference

• feat: WalletConnect v2 integration verification sentrix-labs/sentrix#621 (WalletConnect v2 verification on Sentrix)

Acceptance

• One ConnectButton renders + connects on examples/walletconnect/
• One send-tx works through the connected wallet