MailboxGUI 4.2.1 - Barrel Mailbox Support & Maintenance Polish

MailboxGUI 4.2.1 is a focused maintenance update that adds barrel mailbox support, improves language upgrade handling for older 4.x installs, adds new Notification API helpers, and improves the admin update-check workflow.

Highlights

• Added barrel support for player mailboxes.
• Added barrel support for public Post Office boxes.
• Player mailboxes can now use a single chest or barrel on top of the fence post.
• Post Office boxes can now use a single chest or barrel as the public access container.
• Signs must be attached to the front face of the chest or barrel.
• Existing chest mailbox setups continue working normally.
• Paper new-mail notifications work with player barrel mailboxes.
• Added Notification API helpers for future integrations and addon modules.
• Added /mailbox admin updatecheck and /mba updatecheck.
• Improved update-check output with clearer current/latest version status and the official MailboxGUI download link.
• Improved language upgrade handling for older 4.x installs so old default text can update while customized language text stays protected.

Upgrade Notes

• No mailbox data conversion is required.
• Existing chest mailboxes and Post Office boxes continue working.
• Existing language customizations are preserved where possible.
• Servers upgrading from older 4.x versions receive safer language text updates for command/help wording, typed-recipient wording, and chest/barrel setup wording.