Add Loading Indicators and Enhanced Error Handling to Demo.html

[crStiv]

Description:

This update enhances the user experience in the demo.html file by introducing loading indicators and improving error handling mechanisms for input validation.

Changes:

Loading Indicators:

• Added spinner elements (.loader) for "Generate Proof" and "Verify Proof" buttons.
• Displayed loaders during asynchronous operations.

Enhanced Error Handling:

• Added validation for JSON format and empty inputs.
• Improved error messaging for better debugging.

Code Improvements:

• Ensured consistent formatting and clear logging.
• Minor stylistic and functional refinements for clarity and usability.

These updates aim to improve functionality and responsiveness, making the UI more intuitive and robust.