add save function of explanation

ios/OralTrainer/Views/FullscreenPlayerView.swift

@@ -97,6 +97,7 @@ struct FullscreenPlayerView: View {
         }
         .sheet(item: $explanationSentence) { sentence in
             SentenceExplanationSheet(
+                mediaId: controller.currentItem?.id ?? "",
                 courseTitle: controller.currentItem?.title ?? "口语训练课程",
                 sentence: sentence
             )

ios/OralTrainer/Views/PlayerSectionView.swift

@@ -22,6 +22,7 @@ struct PlayerSectionView: View {
         .padding(.horizontal, 16)
         .sheet(item: $explanationSentence) { sentence in
             SentenceExplanationSheet(
+                mediaId: controller.currentItem?.id ?? "",
                 courseTitle: controller.currentItem?.title ?? "口语训练课程",
                 sentence: sentence
             )
@@ -96,7 +97,7 @@ struct PlayerSectionView: View {
                 Text(sentenceTextText)
                 .font(.system(size: 15))
                     .foregroundColor(.white)
-                    .lineLimit(2)
+                    .lineLimit(4)
                 Spacer()
                 Button {
                     guard let sentence = controller.currentSentence, !(sentence.text ?? "").isEmpty else {
@@ -230,12 +231,14 @@ struct PlayerSectionView: View {
 }
 
 struct SentenceExplanationSheet: View {
+    let mediaId: String
     let courseTitle: String
     let sentence: SentenceBoundary
 
     @Environment(\.dismiss) private var dismiss
     @State private var explanation = ""
     @State private var errorMessage: String?
+    @State private var saved = false
 
     var body: some View {
         NavigationView {
@@ -267,18 +270,35 @@ struct SentenceExplanationSheet: View {
             .navigationBarTitleDisplayMode(.inline)
             .toolbar {
                 ToolbarItem(placement: .navigationBarTrailing) {
-                    Button {
-                        dismiss()
-                    } label: {
-                        Image(systemName: "xmark.circle.fill")
-                            .font(.system(size: 22, weight: .semibold))
+                    HStack(spacing: 6) {
+                        Button(saved ? "已保存" : "保存") {
+                            saveExplanation()
+                        }
+                        .disabled(saved || explanation.isEmpty || errorMessage != nil)
+                        Text(saved ? "（已保存本地）" : "（下次直接看本地）")
+                            .font(.system(size: 12))
                             .foregroundColor(Theme.textMuted)
+                        Button {
+                            dismiss()
+                        } label: {
+                            Image(systemName: "xmark.circle.fill")
+                                .font(.system(size: 22, weight: .semibold))
+                                .foregroundColor(Theme.textMuted)
+                        }
+                        .accessibilityLabel("关闭句子讲解")
                     }
-                    .accessibilityLabel("关闭句子讲解")
                 }
             }
         }
         .task {
+            if let cached = SentenceExplanationStore.shared.load(
+                mediaId: mediaId,
+                sentenceIndex: sentence.index
+            ) {
+                explanation = cached
+                saved = true
+                return
+            }
             guard explanation.isEmpty, errorMessage == nil else { return }
             do {
                 explanation = try await SentenceBoundaryAPI.explain(
@@ -290,4 +310,13 @@ struct SentenceExplanationSheet: View {
             }
         }
     }
+
+    private func saveExplanation() {
+        guard !saved, !explanation.isEmpty, errorMessage == nil else { return }
+        saved = SentenceExplanationStore.shared.save(
+            mediaId: mediaId,
+            sentenceIndex: sentence.index,
+            explanation: explanation
+        )
+    }
 }