修复默写输入框分隔线 crash，改用 HairlineView

LearnEnglish/LearnEnglish/Views/DictationInputView.swift

@@ -5,7 +5,7 @@ final class DictationInputView: UIView, UITextFieldDelegate {
     var onSubmit: (() -> Void)?
 
     private let textField = UITextField()
-    private let bottomLine = UIView()
+    private let bottomLine = HairlineView()
 
     var text: String { textField.text?.trimmingCharacters(in: .whitespaces) ?? "" }
     var isEmpty: Bool { text.isEmpty }
@@ -24,7 +24,6 @@ final class DictationInputView: UIView, UITextFieldDelegate {
         textField.returnKeyType = .done
         textField.addTarget(self, action: #selector(textChanged), for: .editingChanged)
 
-        bottomLine.backgroundColor = .separator
         bottomLine.translatesAutoresizingMaskIntoConstraints = false
 
         addSubview(textField)
@@ -42,7 +41,6 @@ final class DictationInputView: UIView, UITextFieldDelegate {
             bottomLine.leadingAnchor.constraint(equalTo: textField.leadingAnchor),
             bottomLine.trailingAnchor.constraint(equalTo: textField.trailingAnchor),
             bottomLine.bottomAnchor.constraint(equalTo: textField.bottomAnchor),
-            bottomLine.heightAnchor.constraint(equalToConstant: 1.0 / UITraitCollection().displayScale),
         ])
     }