some cleanup and some fixes

lib/src/screens/add_card_screen.dart

@@ -18,11 +18,14 @@ class _AddCardScreenState extends State<AddCardScreen> {
   final _kanaKit = const KanaKit();
   final _deckRepository = CustomDeckRepository();
   bool _useInterval = false;
+  late FocusNode _japaneseFocusNode;
 
   @override
   void initState() {
     super.initState();
     _japaneseController.addListener(_convertToKana);
+    _japaneseFocusNode = FocusNode();
+    _japaneseFocusNode.addListener(_onJapaneseFocusChange);
   }
 
   @override
@@ -31,13 +34,24 @@ class _AddCardScreenState extends State<AddCardScreen> {
     _japaneseController.dispose();
     _englishController.dispose();
     _kanjiController.dispose();
+    _japaneseFocusNode.removeListener(_onJapaneseFocusChange);
+    _japaneseFocusNode.dispose();
     super.dispose();
   }
 
   void _convertToKana() {
     final text = _japaneseController.text;
+    final selection = _japaneseController.selection;
+    final offset = selection.baseOffset;
+
+    if ((offset > 1 && text[offset - 1] == 'n' && text[offset - 2] != 'n') ||
+        (offset == 1 && text[offset - 1] == 'n')) {
+      return;
+    }
+
     final converted = _kanaKit.toKana(text);
-    if (text != converted) {
+
+    if (converted != text) {
       _japaneseController.value = _japaneseController.value.copyWith(
         text: converted,
         selection: TextSelection.fromPosition(
@@ -47,6 +61,21 @@ class _AddCardScreenState extends State<AddCardScreen> {
     }
   }
 
+  void _onJapaneseFocusChange() {
+    if (!_japaneseFocusNode.hasFocus) {
+      _forceNConversion();
+    }
+  }
+
+  void _forceNConversion() {
+    final text = _japaneseController.text;
+    if (text.isNotEmpty &&
+        text.endsWith('n') &&
+        _kanaKit.toKana(text) != text) {
+      _japaneseController.text = _kanaKit.toKana(text);
+    }
+  }
+
   void _saveCard() {
     if (_formKey.currentState!.validate()) {
       final srsData = _useInterval
@@ -83,6 +112,7 @@ class _AddCardScreenState extends State<AddCardScreen> {
             children: [
               TextFormField(
                 controller: _japaneseController,
+                focusNode: _japaneseFocusNode,
                 decoration: const InputDecoration(
                   labelText: 'Japanese (Kana)',
                   hintText: 'Enter Japanese vocabulary or kanji',