add explain module

ios/OralTrainer/Views/PlayerSectionView.swift

@@ -10,6 +10,7 @@ struct PlayerSectionView: View {
 
     @State private var progressDragging = false
     @State private var dragPositionMs: Int64 = 0
+    @State private var explanationSentence: SentenceBoundary?
 
     var body: some View {
         VStack(spacing: 0) {
@@ -19,6 +20,12 @@ struct PlayerSectionView: View {
             }
         }
         .padding(.horizontal, 16)
+        .sheet(item: $explanationSentence) { sentence in
+            SentenceExplanationSheet(
+                courseTitle: controller.currentItem?.title ?? "口语训练课程",
+                sentence: sentence
+            )
+        }
     }
 
     private var playerSurface: some View {
@@ -85,10 +92,33 @@ struct PlayerSectionView: View {
             Text(sentenceMetaText)
                 .font(.system(size: 13))
                 .foregroundColor(Theme.textSubtle)
-            Text(sentenceTextText)
+            HStack(alignment: .center, spacing: 8) {
+                Text(sentenceTextText)
                 .font(.system(size: 15))
-                .foregroundColor(.white)
-                .lineLimit(2)
+                    .foregroundColor(.white)
+                    .lineLimit(2)
+                Spacer()
+                Button {
+                    guard let sentence = controller.currentSentence, !(sentence.text ?? "").isEmpty else {
+                        return
+                    }
+                    controller.pause()
+                    explanationSentence = sentence
+                } label: {
+                    Image(systemName: "lightbulb.circle.fill")
+                        .font(.system(size: 23, weight: .semibold))
+                        .foregroundColor(
+                            controller.currentSentence?.text?.isEmpty == false
+                                ? Theme.accentLight
+                                : Theme.textSubtle
+                        )
+                        .frame(width: 32, height: 32)
+                        .background(Theme.overlay)
+                        .clipShape(Circle())
+                        .accessibilityLabel("AI 讲解这句话")
+                }
+                .buttonStyle(.plain)
+            }
             progressRow
             Text(statusText)
                 .font(.system(size: 13))
@@ -195,3 +225,53 @@ struct PlayerSectionView: View {
         controller.setPlaybackSpeed(next)
     }
 }
+
+struct SentenceExplanationSheet: View {
+    let courseTitle: String
+    let sentence: SentenceBoundary
+
+    @State private var explanation = ""
+    @State private var errorMessage: String?
+
+    var body: some View {
+        NavigationView {
+            ScrollView {
+                VStack(alignment: .leading, spacing: 14) {
+                    Text(sentence.text ?? "")
+                        .font(.system(size: 18, weight: .bold))
+                        .foregroundColor(Theme.textDark)
+                    if let errorMessage {
+                        Text(errorMessage)
+                            .font(.system(size: 15))
+                            .foregroundColor(Theme.danger)
+                    } else if explanation.isEmpty {
+                        Text("AI 正在讲解这句话…")
+                            .font(.system(size: 15))
+                            .foregroundColor(Theme.textSubtle)
+                    } else {
+                        Text(explanation)
+                            .font(.system(size: 16))
+                            .foregroundColor(Theme.textDark)
+                            .lineSpacing(5)
+                    }
+                }
+                .frame(maxWidth: .infinity, alignment: .leading)
+                .padding(20)
+            }
+            .background(Color.white.ignoresSafeArea())
+            .navigationTitle("句子讲解")
+            .navigationBarTitleDisplayMode(.inline)
+        }
+        .task {
+            guard explanation.isEmpty, errorMessage == nil else { return }
+            do {
+                explanation = try await SentenceBoundaryAPI.explain(
+                    sentence,
+                    courseTitle: courseTitle
+                )
+            } catch {
+                errorMessage = "讲解失败：\(error.localizedDescription)"
+            }
+        }
+    }
+}