修复拼写页闪退：改用 becomeFirstResponder 管理焦点

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

LearnEnglish/LearnEnglish/Views/SpellingWordInput.swift

@@ -5,6 +5,7 @@ import UIKit
 struct KeystrokeCaptureField: UIViewRepresentable {
     let onChar: (Character) -> Void
     let onBackspace: () -> Void
+    @Binding var isActive: Bool
 
     func makeUIView(context: Context) -> UITextField {
         let tf = UITextField()
@@ -19,7 +20,13 @@ struct KeystrokeCaptureField: UIViewRepresentable {
         Coordinator(onChar: onChar, onBackspace: onBackspace)
     }
 
-    func updateUIView(_ uiView: UITextField, context: Context) {}
+    func updateUIView(_ uiView: UITextField, context: Context) {
+        if isActive {
+            uiView.becomeFirstResponder()
+        } else {
+            uiView.resignFirstResponder()
+        }
+    }
 
     class Coordinator: NSObject, UITextFieldDelegate {
         let onChar: (Character) -> Void
@@ -61,7 +68,7 @@ struct SpellingWordInput: View {
     let blankPositions: [Int]
     @Binding var currentBlankIndex: Int
     @Binding var filledBlanks: [String]
-    @FocusState private var isFocused
+    @State private var isFocused = false
 
     private let chars: [String]
 
@@ -85,11 +92,10 @@ struct SpellingWordInput: View {
                     guard currentBlankIndex > 0 else { return }
                     currentBlankIndex -= 1
                     filledBlanks[currentBlankIndex] = ""
-                }
+                },
+                isActive: $isFocused
             )
             .frame(width: 0, height: 0)
-            .opacity(0.01)
-            .focused($isFocused)
 
             HStack(spacing: 4) {
                 ForEach(Array(chars.enumerated()), id: \.offset) { index, ch in