content [nfc]: Pull out child variable in MessageContent.build

For the edit-message feature, the Figma fades message content during
the loading state:
  https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=4026-8563&m=dev
This refactor prepares for that.

Author: chrisbobbe

lib/widgets/content.dart

@@ -296,13 +296,16 @@ class MessageContent extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final content = this.content;
+
+    Widget child = switch (content) {
+      ZulipContent() => BlockContentList(nodes: content.nodes),
+      PollContent()  => PollWidget(messageId: message.id, poll: content.poll),
+    };
+
     return InheritedMessage(message: message,
       child: DefaultTextStyle(
         style: ContentTheme.of(context).textStylePlainParagraph,
-        child: switch (content) {
-          ZulipContent() => BlockContentList(nodes: content.nodes),
-          PollContent()  => PollWidget(messageId: message.id, poll: content.poll),
-        }));
+        child: child));
   }
 }