feat: unread message tracking with blue dot indicator

Add ContactThreadRead model to track when users last viewed each contact thread.
Contacts with messages newer than the last read time show a blue dot in the sidebar.
Opening a thread automatically marks it as read via markThreadRead mutation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/graphql/operations/contacts.graphql

@@ -7,6 +7,7 @@ query ContactsQuery {
     lastContactAt
     lastMessageText
     lastMessageChannel
+    hasUnread
     description
   }
 }