Refactor form data handling in index.html

[imanchalsingh]

Added error handling and minor improvements to JS form submission for safer and cleaner data posting.

[Copilot]

Pull Request Overview

This PR refactors the form submission logic in the postMultipartFormData example to improve code quality and error handling. The changes modernize the JavaScript syntax and add better user feedback mechanisms.

Key changes:

• Modernized variable declarations from var to const
• Improved nested object handling to preserve existing properties
• Enhanced error handling with console logging and user feedback
• Conditional Content-Type header based on data format

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