Enhancement: Improved File Upload Experience

[nikpivkin]

Currently, when the default channel is not selected in the settings, the UI displays an error when attempting to upload a file, but does not provide information about the cause of this error.

Description of improvements:

• If a user tries to upload a file without selecting the default channel, show a warning message. This can be a popup, a message on the screen, or another element that catches the user's attention.
• If no default channel is selected, make the download button inactive (greyed out or otherwise marked as unavailable). This signals users that a preliminary step must be completed before the file can be downloaded.