Describe the bug
The BaseWalletConnectionButton component in @solana/wallet-adapter-react-ui overrides custom className props with its default class. When developers pass a custom className to style the button, it gets replaced by the default wallet-adapter-button-trigger class instead of being combined with it.

To Reproduce

1. Import BaseWalletConnectionButton from @solana/wallet-adapter-react-ui
2. Use the component with a custom className prop:

<BaseWalletConnectionButton className="my-custom-class" />

3. Inspect the rendered button element
4. Observe that only wallet-adapter-button-trigger class is present, and my-custom-class is missing

Expected behavior
The component should preserve both the custom className and the default class. The rendered button should have both classes:

<button class="my-custom-class wallet-adapter-button-trigger">

Screenshots
N/A

Desktop (please complete the following information):

• This is a component library issue that affects all environments where the library is used
• Version: 0.9.35

Smartphone (please complete the following information):

• Not applicable as this is a component library issue

Additional context
Current implementation in BaseWalletConnectionButton.tsx:

<Button
    {...props}
    className={`wallet-adapter-button-trigger`}
    startIcon={...}
/>

Proposed fix:

export function BaseWalletConnectionButton({ walletIcon, walletName, className, ...props }: Props) {
    return (
        <Button
            {...props}
            className={`${className || ''} wallet-adapter-button-trigger`}
            startIcon={...}
        />
    );
}

This is a common pattern in React components where we want to allow customization while maintaining base functionality. The current implementation prevents developers from applying custom styles through className props.