welcome message: link @zio with a mention entity

internal/domain/welcome_message.go

@@ -10,6 +10,14 @@ import (
 	"time"
 )
 
+// officialUpdatesChannelMention is the public @username of the updates channel
+// linked from the welcome message when the resolved body includes it. It is
+// carried in a MessageEntityMention wherever it appears in body so clients
+// render it as a tappable link; the mention itself lives in the default
+// templates in login_welcome_template.go (or an admin-panel override), not
+// hardcoded here.
+const officialUpdatesChannelMention = "@zio"
+
 // OfficialWelcomeMessage builds the account-visible incoming message sent
 // from the official system account on every completed sign-in (SignUp and
 // every subsequent SignIn/SignInWithEmail), regardless of delivery channel.
@@ -28,13 +36,42 @@ func OfficialWelcomeMessage(userID int64, body string, date int) (Message, error
 	if userID <= 0 || IsSystemUserID(userID) || body == "" || date < 0 || date > math.MaxInt32 {
 		return Message{}, fmt.Errorf("%w: user=%d date=%d", ErrLoginCodeDeliveryInvalid, userID, date)
 	}
-	return Message{
+	msg := Message{
 		OwnerUserID: userID,
 		Peer:        Peer{Type: PeerTypeUser, ID: OfficialSystemUserID},
 		From:        Peer{Type: PeerTypeUser, ID: OfficialSystemUserID},
 		Date:        date,
 		Body:        body,
-	}, nil
+	}
+	if ent, ok := usernameMentionEntity(body, officialUpdatesChannelMention); ok {
+		msg.Entities = []MessageEntity{ent}
+	}
+	return msg, nil
+}
+
+// usernameMentionEntity locates mention ("@name") in text and returns a
+// MessageEntityMention spanning it, with the UTF-16 offset/length clients expect.
+func usernameMentionEntity(text, mention string) (MessageEntity, bool) {
+	before, _, found := strings.Cut(text, mention)
+	if !found {
+		return MessageEntity{}, false
+	}
+	return MessageEntity{
+		Type:   MessageEntityMention,
+		Offset: utf16Len(before),
+		Length: utf16Len(mention),
+	}, true
+}
+
+func utf16Len(s string) int {
+	n := 0
+	for _, r := range s {
+		n++
+		if r > 0xffff {
+			n++
+		}
+	}
+	return n
 }
 
 // SignInMethodLabel returns the human-readable method name embedded in