feat: improve AI dialogue evaluation and capability boundaries

kouyu_english/lib/widgets/lexicon_lookup.dart

@@ -28,8 +28,9 @@ List<VocabularyItem> get courseLexiconEntries {
     ...allLessons.expand((lesson) => lesson.vocabulary),
   ];
   final seen = <String>{};
-  _cachedEntries = entries.where((item) => seen.add(item.word.toLowerCase())).toList()
-    ..sort((a, b) => b.word.length.compareTo(a.word.length));
+  _cachedEntries =
+      entries.where((item) => seen.add(item.word.toLowerCase())).toList()
+        ..sort((a, b) => b.word.length.compareTo(a.word.length));
   return _cachedEntries!;
 }
 
@@ -385,12 +386,13 @@ class _LexiconLookupSheetState extends State<_LexiconLookupSheet> {
       sentenceAnalysisError = null;
     });
 
-    final result = await AiService.instance.analyzeSentence(
-      provider: widget.state.aiProvider,
-      endpoint: widget.state.aiEndpoint,
-      model: widget.state.aiModel,
-      text: text,
-    );
+    final result = await AiService.instance.capabilities.lexicon
+        .analyzeSentence(
+          provider: widget.state.aiProvider,
+          endpoint: widget.state.aiEndpoint,
+          model: widget.state.aiModel,
+          text: text,
+        );
 
     if (!mounted) return;
     setState(() {
@@ -418,7 +420,7 @@ class _LexiconLookupSheetState extends State<_LexiconLookupSheet> {
       requestingTemporaryDefinition = true;
       temporaryError = null;
     });
-    final definition = await AiService.instance.temporaryDefinition(
+    final definition = await AiService.instance.capabilities.lexicon.define(
       provider: widget.state.aiProvider,
       endpoint: widget.state.aiEndpoint,
       model: widget.state.aiModel,

kouyu_english/lib/widgets/voice_answer.dart

@@ -76,7 +76,7 @@ mixin VoiceAnswerMixin<T extends StatefulWidget> on State<T> {
     });
     if (path == null) return;
     final config = voiceState.aiConfig;
-    final transcribed = await AiService.instance.transcribeAudio(
+    final transcribed = await AiService.instance.capabilities.speech.transcribe(
       filePath: path,
       provider: config.provider,
       endpoint: config.endpoint,