Export as `.txt` File 📄

[Sappymukherjee214]

🚀 Is your feature request related to a problem? Please describe.

Currently, the platform only supports exporting humanized content as PDF and Word (.docx). While these are common formats, many users (especially developers or minimalists) prefer a simple .txt file that is lightweight and easy to open in any generic text editor without specialized software.

💡 Describe the solution you'd like

I would like to add a third export option to the "Export" component in the dashboard. This includes:

• Updating the UI in frontend/src/App.tsx (the handleExport function and the header buttons) to add a button for Export TXT.
• Modifying the backend/src/controllers/exportController.ts to handle the .txt format request and correctly set the Content-Type: text/plain header.
• Ensuring the generated file is downloadable with the correct filename and extension.

🎨 Describe alternatives you've considered

An alternative is for users to manually copy and paste the text into a .txt file, but a built-in export feature provides a much more professional and convenient developer experience.

📝 Additional context

This should be implemented using the existing export logic, simply adding a new condition to handle the text/plain MIME type in the response.

[Sappymukherjee214]

@HuiNeng6 this is not for you, please close this pull request