Surface logo upload errors to user in server settings

Summary

When uploading a new logo in the server settings UI, errors were silently ignored and the user was redirected back to the settings page without any feedback.

Changes

• Added Django messages framework import to
• Updated to show success/error messages based on form validation
• Updated to display Django messages using the messages framework
• Renamed variable to to avoid shadowing Django's settings module

Testing

• Django system checks pass ()
• The template now properly displays success/error messages from the messages framework