Update index.ts to expose SimpleWalletClient object

[roschler]

The SimpleWalletClient object is needed by developers creating Web3 apps that interact with the user's wallet. For example:

    const config: StoryConfig = {
        chainId: await this.getCurrentChainId(), // Get the current chain ID from MetaMask
        transport: http(RPCProviderUrl), // Using MetaMask's provider
        wallet: this.walletClient as SimpleWalletClient
    };

This is so they can interact with the user's wallet for doing important operations on the Story Protocol blockchain, like signing payloads or paying for transactions.

Description

Example:
This pr adds user login function, includes：

• 1. add user login page.
• 2. ...

Test Plan

Example:

• 1. Use different test accounts for login tests, including correct user names and passwords, and incorrect user names and passwords.
• 2. ...

Related Issue

Example: Issue #123

Notes

• Example: Links and navigation need to be added to the front-end interface