simplify: dedupe boilerplate and remove dead code across all layers

lib/view/pages/marianum_message/search_marianum_messages.dart

@@ -2,6 +2,7 @@ import 'package:flutter/material.dart';
 
 import '../../../routing/app_routes.dart';
 import '../../../state/app/modules/marianum_message/bloc/marianum_message_state.dart';
+import '../../../widget/centered_leading.dart';
 import '../../../widget/placeholder_view.dart';
 
 class SearchMarianumMessages extends SearchDelegate<MarianumMessage?> {
@@ -45,10 +46,7 @@ class SearchMarianumMessages extends SearchDelegate<MarianumMessage?> {
       itemBuilder: (_, i) {
         final message = matches[i];
         return ListTile(
-          leading: const Column(
-            mainAxisAlignment: MainAxisAlignment.center,
-            children: [Icon(Icons.newspaper)],
-          ),
+          leading: const CenteredLeading(Icon(Icons.newspaper)),
           title: Text(message.name, overflow: TextOverflow.ellipsis),
           subtitle: Text('vom ${message.date}'),
           trailing: const Icon(Icons.arrow_right),