Fix signer for coinbase

[humanagent]

Refactor XMTP client initialization and signer creation in WalletConnection component to support Warpcast API proxy integration

• Centralizes signer creation logic in new getSigner function within WalletConnection.tsx that handles ephemeral, EOA, and Coinbase Smart Wallet types
• Replaces createSignerForCoinbaseSmartWallet with createOnchainKitSigner in xmtp.ts to provide direct signature handling without ECDSA-like signature generation
• Separates XMTP initialization from wallet connection logic through a new effect hook that triggers on connection type or wallet availability changes

📍Where to Start

Start with the getSigner function in WalletConnection.tsx which centralizes the signer creation logic for all wallet types.

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Macroscope summarized 3000cbc.