[Component] Add checkbox component

Currently only supports text input. A checkbox would be useful for boolean flags.

**Example usage:**
```yaml
- checkbox:
    label: "Remember me"
    checked: true
```

**What's needed:**
- Add `Checkbox` class in `abstract_classes/`
- Add to `COMPONENT_MAP` in cli.py
- Add `Checkbox.j2` template for each backend (html, curses, imgui)