feat(IonTabButton): publicly available el

### Prerequisites

- [x] I have read the [Contributing Guidelines](https://github.com/ionic-team/ionic-framework/blob/main/docs/CONTRIBUTING.md#creating-an-issue).
- [x] I agree to follow the [Code of Conduct](https://ionicframework.com/code-of-conduct).
- [x] I have searched for [existing issues](https://github.com/ionic-team/ionic-framework/issues) that already include this feature request, without success.

### Describe the Feature Request

I would like `el` to be publicly available

### Describe the Use Case

To add this little blue bar, and to animate it right and left I need to get the active tab element reference, then to extract his `el`, find his position and them with a bit of math move that little bar :)

https://github.com/user-attachments/assets/52ee9af0-ddc3-43ac-a21b-6c7c5fca1489

### Describe Preferred Solution

Make the `el` being publicly available

### Describe Alternatives

_No response_

### Related Code

_No response_

### Additional Information

I'd be more than happy to do a PR if this issue gets approved

Start by locating the IonTabButton component and its public API entry point in the Ionic framework. Determine how the active tab element reference is exposed and verify that its el property is publicly accessible without breaking existing tab behavior; done means consumers can retrieve the element for positioning and animation.

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