ui: Center-align actual text (not just column children) on beta homescreen

Fixes: #428

Author: chrisbobbe

lib/widgets/app.dart

@@ -230,6 +230,7 @@ class HomePage extends StatelessWidget {
       body: Center(
         child: Column(mainAxisAlignment: MainAxisAlignment.center, children: [
           DefaultTextStyle.merge(
+            textAlign: TextAlign.center,
             style: const TextStyle(fontSize: 18),
             child: Column(children: [
               const Text('🚧 Under construction 🚧'),