users [nfc]: Use userDisplayName at last non-self-user sites in widgets/

We've now centralized on store.userDisplayName and
store.senderDisplayName for all the code that's responsible for
showing a user's name on the screen, except for a few places we use
`User.fullName` for (a) the self-user and (b) to create an @-mention
for the compose box. The "(unknown user)" and upcoming "Muted user"
placeholders aren't needed for (a) or (b).

Author: chrisbobbe

lib/widgets/compose_box.dart

@@ -858,9 +858,10 @@ class _FixedDestinationContentInput extends StatelessWidget {
 
       case DmNarrow(otherRecipientIds: [final otherUserId]):
         final store = PerAccountStoreWidget.of(context);
-        final fullName = store.getUser(otherUserId)?.fullName;
-        if (fullName == null) return zulipLocalizations.composeBoxGenericContentHint;
-        return zulipLocalizations.composeBoxDmContentHint(fullName);
+        final user = store.getUser(otherUserId);
+        if (user == null) return zulipLocalizations.composeBoxGenericContentHint;
+        return zulipLocalizations.composeBoxDmContentHint(
+          store.userDisplayName(otherUserId));
 
       case DmNarrow(): // A group DM thread.
         return zulipLocalizations.composeBoxGroupDmContentHint;

lib/widgets/profile.dart

@@ -49,7 +49,7 @@ class ProfilePage extends StatelessWidget {
       Center(
         child: Avatar(userId: userId, size: 200, borderRadius: 200 / 8)),
       const SizedBox(height: 16),
-      Text(user.fullName,
+      Text(store.userDisplayName(userId),
         textAlign: TextAlign.center,
         style: _TextStyles.primaryFieldText
           .merge(weightVariableTextStyle(context, wght: 700))),
@@ -75,7 +75,7 @@ class ProfilePage extends StatelessWidget {
     ];
 
     return Scaffold(
-      appBar: ZulipAppBar(title: Text(user.fullName)),
+      appBar: ZulipAppBar(title: Text(store.userDisplayName(userId))),
       body: SingleChildScrollView(
         child: Center(
           child: ConstrainedBox(