Add theme configuration support to ApolloServerPluginLandingPageLocalDefault

### Feature Request

**Is your feature request related to a problem? Please describe.**

Currently, `ApolloServerPluginLandingPageProductionDefault` supports theme configuration (light/dark mode) via the `document` option, but `ApolloServerPluginLandingPageLocalDefault` does not offer this capability.

**Describe the solution you'd like**

I would like `ApolloServerPluginLandingPageLocalDefault` to support the same `document` configuration option as `ApolloServerPluginLandingPageProductionDefault`:

Example usage:
```typescript
plugins: [
ApolloServerPluginLandingPageLocalDefault({
initialState: {
displayOptions: {
theme: 'dark', // or 'light'
}
},
}),
]
```

**Describe alternatives you've considered**

1. **Using Production Landing Page in Development**: I specifically do not want to rely on production landing page as it require to login to studio apollo graphql, while I only use this page for local development.

**Additional context**

- The production landing page already has this feature implemented, suggesting the underlying infrastructure exists
- This appears to be a straightforward feature parity issue rather than a fundamental architectural limitation

Compare the ApolloServerPluginLandingPageLocalDefault and ApolloServerPluginLandingPageProductionDefault entry points, then inspect the existing production landing-page tests for document or theme configuration. Confirm whether the requested local option is document or initialState, add equivalent local support, and update tests to cover light and dark themes.

Written by the indexing model from the issue text.