msglist: Fix DM header color lerp bug.

Fixed a bug in MessageListTheme.lerp() where dmRecipientHeaderBg was using
streamMessageBgDefault instead of dmRecipientHeaderBg for interpolation.

Author: E-m-i-n-e-n-c-e

lib/widgets/message_list.dart

@@ -153,7 +153,7 @@ class MessageListTheme extends ThemeExtension<MessageListTheme> {
     return MessageListTheme._(
       dateSeparator: Color.lerp(dateSeparator, other.dateSeparator, t)!,
       dateSeparatorText: Color.lerp(dateSeparatorText, other.dateSeparatorText, t)!,
-      dmRecipientHeaderBg: Color.lerp(streamMessageBgDefault, other.dmRecipientHeaderBg, t)!,
+      dmRecipientHeaderBg: Color.lerp(dmRecipientHeaderBg, other.dmRecipientHeaderBg, t)!,
       messageTimestamp: Color.lerp(messageTimestamp, other.messageTimestamp, t)!,
       recipientHeaderText: Color.lerp(recipientHeaderText, other.recipientHeaderText, t)!,
       senderBotIcon: Color.lerp(senderBotIcon, other.senderBotIcon, t)!,