[XB] Private Threads 2.1.0

Let users create threads visible only to invited members.

Features:​

• Custom Titles: Prepend/append text to private thread titles.
• Thread Privacy Control: Convert existing threads to private and vice versa.
• User Alerts: Notify members when they’re invited to a private thread.
• Member Management: Add/remove users after thread creation.
• Access Requests (Optional): Allow users to request access to private threads.
• Upgrade Options: Users can purchase an extension to increase private thread limits.
• Default Invitations: Set a predefined list of users invited to all new private threads.
• Visibility Control: Option to show private threads in the thread list.
• Invitation Permissions: Control who can be invited to private threads.
• Admin/Mod Access: Permission to view any private thread, with an option for admins to see private threads even without permission (off by default).