msglist: In single-conversation view, make recipient headers not tapp…

…able.

Updated onTap for recipient headers in single-conversation view.
Adjusted GestureDetector logic to conditionally enable navigation.
Simplified ColoredBox structure for non-tappable recipient headers.
Improves user experience by removing unnecessary tap interactions.

lib/widgets/message_list.dart

@@ -1105,9 +1105,9 @@ class StreamMessageRecipientHeader extends StatelessWidget {
         ]));
 
     return GestureDetector(
-      onTap: () => Navigator.push(context,
+      onTap: () => showStream? Navigator.push(context,
         MessageListPage.buildRoute(context: context,
-          narrow: TopicNarrow.ofMessage(message))),
+          narrow: TopicNarrow.ofMessage(message))): null,
       onLongPress: () => showTopicActionSheet(context,
         channelId: message.streamId, topic: topic),
       child: ColoredBox(
@@ -1148,29 +1148,25 @@ class DmRecipientHeader extends StatelessWidget {
 
     final messageListTheme = MessageListTheme.of(context);
 
-    return GestureDetector(
-      onTap: () => Navigator.push(context,
-        MessageListPage.buildRoute(context: context,
-          narrow: DmNarrow.ofMessage(message, selfUserId: store.selfUserId))),
-      child: ColoredBox(
-        color: messageListTheme.dmRecipientHeaderBg,
-        child: Padding(
-          padding: const EdgeInsets.symmetric(vertical: 11),
-          child: Row(
-            crossAxisAlignment: CrossAxisAlignment.center,
-            children: [
-              Padding(
-                padding: const EdgeInsets.symmetric(horizontal: 6),
-                child: Icon(
-                  color: messageListTheme.recipientHeaderText,
-                  size: 16,
-                  ZulipIcons.user)),
-              Expanded(
-                child: Text(title,
-                  style: recipientHeaderTextStyle(context),
-                  overflow: TextOverflow.ellipsis)),
-              RecipientHeaderDate(message: message),
-            ]))));
+    return ColoredBox(
+      color: messageListTheme.dmRecipientHeaderBg,
+      child: Padding(
+        padding: const EdgeInsets.symmetric(vertical: 11),
+        child: Row(
+          crossAxisAlignment: CrossAxisAlignment.center,
+          children: [
+            Padding(
+              padding: const EdgeInsets.symmetric(horizontal: 6),
+              child: Icon(
+                color: messageListTheme.recipientHeaderText,
+                size: 16,
+                ZulipIcons.user)),
+            Expanded(
+              child: Text(title,
+                style: recipientHeaderTextStyle(context),
+                overflow: TextOverflow.ellipsis)),
+            RecipientHeaderDate(message: message),
+          ])));
   }
 }