fix(app): resolve variable redeclaration syntax error (#899)

[Nicks-19]

Related Issue

Closes #899

Summary

This pull request resolves a critical JavaScript syntax error thrown on page load by removing a duplicate variable declaration in the frontend application script.

Changes Made

• Removed the duplicate const calendarDownloadBtn declaration on line 1384 of js/app.js to resolve the browser crash.
• Retained the original declaration on line 89 to bind the element and handle the calendar download click event listener.

Testing

• Verified syntactical validity of the modified script using Node.js syntax compiler: node -c js/app.js (successfully parsed without errors).
• Verified programmatically using a custom file scanner script to confirm that calendarDownloadBtn is declared exactly once in the file.

Checklist

• Code follows project style
• Tested locally
• No unrelated changes included
• Documentation updated (if applicable)