大小写切换改为仅刷新当前字母显示，不再重置整个页面

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

LearnEnglish/LearnEnglish/Views/QuizViewController.swift

@@ -425,7 +425,15 @@ final class QuizViewController: UIViewController {
         isUppercase.toggle()
         UserDefaults.standard.set(isUppercase, forKey: "spelling_uppercase")
         updateCaseButton()
-        restart()
+        selectedOption = nil
+        let words = config.words.map { word in
+            let text = isUppercase ? word.word.uppercased() : word.word.lowercased()
+            return Word(categoryId: word.categoryId, word: text, phonetic: word.phonetic,
+                        meaning: word.meaning, example: word.example, exampleMeaning: word.exampleMeaning)
+        }
+        questions = QuizGenerator.makeQuestions(mode: config.mode, words: words)
+        showQuestion()
+        spellingView.activate()
     }
 
     private func updateCaseButton() {