home: Use weightVariableTextStyle instead of TextStyle.fontWeight

Now, this text is styled with a `wght` value of 700, instead of 400
from the ambient style. The FontWeight.bold seems to have already
been causing the spans to render more boldly than surrounding text,
but in a way that doesn't match what we get for Source Code Pro with
`wght` 700. I think this is because of
  flutter/flutter#136779
.

We could probably have left out the `const TextStyle().merge()`; I
think the only difference it causes is in the resulting style's
debugLabel. But I figured we might as well be consistent in how we
use weightVariableTextStyle, and in any case it should disappear
with
  flutter/flutter#148026 .

Author: chrisbobbe

lib/widgets/app.dart

@@ -15,6 +15,7 @@ import 'page.dart';
 import 'recent_dm_conversations.dart';
 import 'store.dart';
 import 'subscription_list.dart';
+import 'text.dart';
 import 'theme.dart';
 
 class ZulipApp extends StatefulWidget {
@@ -242,7 +243,8 @@ class HomePage extends StatelessWidget {
     final zulipLocalizations = ZulipLocalizations.of(context);
 
     InlineSpan bold(String text) => TextSpan(
-      text: text, style: const TextStyle(fontWeight: FontWeight.bold));
+      style: const TextStyle().merge(weightVariableTextStyle(context, wght: 700)),
+      text: text);
 
     int? testStreamId;
     if (store.connection.realmUrl.origin == 'https://chat.zulip.org') {