feat: Convert index.html to React application and refactor components

[mibracy]

This PR converts the existing index.html to a React application. All HTML structure, CSS styling, and JavaScript logic have been migrated to React components. The original index.html file has been removed.

[mibracy]

This Pull Request addresses and closes issue #2: "Convert index.html to a React application".

[mibracy]

This PR has been updated with several key fixes and improvements:

• Resolved Rendering Crashes: Fixed multiple ReferenceError issues (e.g., role is not defined, id is not defined) by correctly escaping JSX placeholders in <h3> tags across Admin.jsx, Agent.jsx, and Customer.jsx.
• Enhanced Robustness: Implemented comprehensive optional chaining (?.) and fallback values (|| '') for all API-derived data access within JSX to prevent null-pointer exceptions during initial component renders.
• Visual Fidelity Adjustments: Corrected CSS to ensure the React application's visual appearance closely matches the original index.html.
  • Adapted div border rules to target specific content blocks (.menu, .model-section, #response-container, .form-group-item, .form-group-grid) to avoid unwanted borders while maintaining intended visual structure.
  • Ensured consistent button padding by adding box-sizing: border-box;.
• General Refinements: Cleaned up useEffect anti-patterns and ensured idiomatic React input handling for numeric fields.