treewide: dart format + analyze + some other fixes

lib/widgets/kanjimaji.dart

@@ -1,15 +1,8 @@
 import 'package:flutter/material.dart';
 
-extension _Hexcode on Color {
-  String get hexcode => '#${value.toRadixString(16).padLeft(8, '0')}';
-}
-
 class Kanimaji extends StatelessWidget {
   final String kanji;
-  const Kanimaji({
-    Key? key,
-    required this.kanji,
-  }) : super(key: key);
+  const Kanimaji({super.key, required this.kanji});
 
   @override
   Widget build(BuildContext context) {