SSTV directory fixes

[patrickrb]

resolves #3062

This pull request primarily focuses on enhancing the file and directory handling in the application. The changes include an update to the folderSize function in storage_helper.php to handle cases where the input is not a directory or if the directory is missing, and the addition of an index.html file in assets/sstvimages to prevent directory access and also adds the sstvimages directory to the repo.

Here are the key changes:

File and Directory Handling:

• application/helpers/storage_helper.php: The folderSize function now checks if the input $dir is a directory before proceeding with its operations. If $dir is not a directory, the function returns 0. This change helps prevent errors that could occur when the function is called with an invalid directory. [1] [2]
• assets/sstvimages/index.html: A new index.html file has been added to the assets/sstvimages directory. This file displays a "403 Forbidden" message, effectively preventing unauthorized directory access. Adding this file to the directory also allows us to add the sstvimages directory to the repo and should prevent users from not having it in the future.

[phl0]

Works for me and resolves #3062.