fix: Add model size warning and improve model loading

[TasadduqB]

Problem

The current implementation does not provide a clear indication of the model size and download time, leading to a poor user experience.

Solution

This PR adds a model size warning to the demo page and improves the model loading process by returning a promise.

Testing

To test this PR, please verify that the model size warning is displayed correctly and that the model is loaded successfully.

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Closes #65

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[tomayac]

Please make the google-cla bot happy, then @maudnals can review.

[tomayac]

@TasadduqB can you make our @google-cla bot happy? You need to sign a Contributor License Agreement (CLA).