remove unused dead code files

lib/widget/about/about.dart

@@ -1,15 +0,0 @@
-import 'package:flutter/material.dart';
-
-class About extends StatelessWidget {
-  const About({super.key});
-
-  @override
-  Widget build(BuildContext context) => Scaffold(
-    appBar: AppBar(title: const Text('Über diese App')),
-    body: const Card(
-      elevation: 1,
-      borderOnForeground: true,
-      child: Text('Marianum Fulda'),
-    ),
-  );
-}

lib/widget/string_extensions.dart

@@ -1,4 +0,0 @@
-extension StringExtensions on String {
-  String capitalize() =>
-      '${this[0].toUpperCase()}${substring(1).toLowerCase()}';
-}

lib/widget/unimplemented_dialog.dart

@@ -1,11 +0,0 @@
-import 'package:flutter/material.dart';
-
-class UnimplementedDialog {
-  static void show(BuildContext context) {
-    showDialog(
-      context: context,
-      builder: (context) =>
-          const AlertDialog(content: Text('Not implemented yet')),
-    );
-  }
-}