Add calculator demo app using Material Design Lite

[dev-zohaibkhan]

This PR adds a beginner-friendly calculator demo app using Material Design Lite components.
The project is located in demos/calculator-app and demonstrates a functional calculator UI.

[google-cla]

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[dev-zohaibkhan]

@googlebot cla

[dev-zohaibkhan]

Hello, I have signed the Individual CLA.
This workflow is awaiting approval as I am a first-time contributor.
Could you please approve the workflow run? Thank you!

[dev-zohaibkhan]

Hello, I have signed the Google Individual CLA.

This workflow is currently awaiting approval because I am a first-time contributor.

Could you please approve the workflow run so the CLA check can complete?

Thank you for your time and support!