Adds the ability to enable the Code Interpreter feature by appending
`code-interpreter=true` to the chat URL.
The `Chat.svelte` component's `initNewChat` function has been updated
to parse this URL parameter. If present, it sets the
`codeInterpreterEnabled` state variable to true.
This change leverages the existing mechanisms for:
- Saving and restoring `codeInterpreterEnabled` state to/from localStorage.
- Displaying and toggling the Code Interpreter state via the existing UI
button in the `MessageInput` component.
No new UI elements were added as the existing UI correctly reflects
the feature's state.
