Feat: No Quit/Cancel Button in Debate Setup

### Feature

### There is currently no **Quit/Cancel** button available in the **Debate Setup** modal.

If a user enters the debate setup but decides not to participate, there is no clear option to exit or cancel the debate. The only available action is **"I'm Ready"**, which proceeds with the debate setup.

## Expected Behavior

A **Quit**, **Cancel**, or **Leave Debate** button should be provided so that users can exit the debate setup without starting the debate.

## Suggested Solution

Add a secondary button such as:
- **Leave Debate**

The button could be placed next to the **"I'm Ready"** button or in the top-right corner of the modal.

## Screenshot

The current Debate Setup modal does not provide any option to cancel or quit the debate.

### Additional Context

### Code of Conduct

- [x] I have joined the [Discord server](https://discord.gg/hjUhu33uAn) and will post updates there
- [x] I have searched existing issues to avoid duplicates

Locate the Debate Setup modal and its “I'm Ready” action in the TypeScript frontend. Add a visible cancel or leave action that exits without starting the debate, then run the relevant frontend checks and confirm both actions behave correctly.

索引モデルが issue の本文から書いたものです。