add explain module

ios/OralTrainer/Views/FullscreenPlayerView.swift

@@ -6,6 +6,7 @@ struct FullscreenPlayerView: View {
     let onExit: () -> Void
 
     @State private var subtitleVisible = true
+    @State private var explanationSentence: SentenceBoundary?
 
     var body: some View {
         ZStack {
@@ -38,10 +39,34 @@ struct FullscreenPlayerView: View {
                         Text(sentenceMetaText)
                             .font(.system(size: 13))
                             .foregroundColor(Theme.textOverlay)
-                        Text(sentenceTextText)
+                        HStack(alignment: .center, spacing: 8) {
+                            Text(sentenceTextText)
                             .font(.system(size: 15))
                             .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: 21, weight: .semibold))
+                                    .foregroundColor(
+                                        controller.currentSentence?.text?.isEmpty == false
+                                            ? Theme.accentLight
+                                            : Theme.textSubtle
+                                    )
+                                    .frame(width: 30, height: 30)
+                                    .background(Theme.overlay)
+                                    .clipShape(Circle())
+                                    .accessibilityLabel("AI 讲解这句话")
+                            }
+                            .buttonStyle(.plain)
+                        }
                         progressRow
                         Text(statusText)
                             .font(.system(size: 13))
@@ -70,6 +95,12 @@ struct FullscreenPlayerView: View {
             }
             .padding(16)
         }
+        .sheet(item: $explanationSentence) { sentence in
+            SentenceExplanationSheet(
+                courseTitle: controller.currentItem?.title ?? "口语训练课程",
+                sentence: sentence
+            )
+        }
     }
 
     @State private var progressDragging = false