Read Receipts para Mattermost

Mattermost has no read receipts in any edition. This plugin adds them only on 1:1 direct messages: a grey tick on send and a blue double tick when the other person has read. It stores no state: it uses the `last_viewed_at` Mattermost already tracks, publishes a WebSocket event on read and paints the ticks on web and desktop. Go server, TypeScript client. Compatible with Mattermost Server v11.7.0 or later, Team or Enterprise, no extra licence. Upload the `.tar.gz` from Releases.