add a refresh action

ios/OralTrainer/Views/CatalogView.swift

@@ -10,6 +10,7 @@ final class CatalogViewModel: ObservableObject {
 
     private var baseURL = AppConfig.serverBaseURL
     private var hasLoaded = false
+    private var boundaryDocuments: [String: SentenceBoundaryDocument] = [:]
 
     func loadIfNeeded() async {
         guard !hasLoaded else {
@@ -31,6 +32,38 @@ final class CatalogViewModel: ObservableObject {
         }
     }
 
+    func preloadBoundaries(for videos: [TrainingVideoSummary]) async -> (
+        videos: [TrainingVideoSummary],
+        succeeded: Int,
+        failed: Int
+    ) {
+        var updatedVideos = videos
+        var succeeded = 0
+        var failed = 0
+
+        for (index, video) in videos.enumerated() {
+            do {
+                let document = try await SentenceBoundaryAPI.fetch(videoHash: video.videoHash)
+                boundaryDocuments[document.videoHash] = document
+                updatedVideos[index] = video.copy(
+                    durationMs: document.durationMs,
+                    sentenceCount: document.sentences.count
+                )
+                if activeVideoHash == video.videoHash {
+                    controller?.updateBoundaries(document.sentences)
+                    controller?.boundaryState = .loaded(count: document.sentences.count)
+                    controller?.showTransientMessage("课程已就绪：\(document.sentences.count) 句")
+                }
+                succeeded += 1
+            } catch {
+                failed += 1
+            }
+        }
+
+        videos = updatedVideos
+        return (updatedVideos, succeeded, failed)
+    }
+
     /// 与安卓端 loadRemoteVideo() 一致：先以无句边界装载，再拉取句边界后重新装载。
     func select(_ video: TrainingVideoSummary) {
         guard let controller = controller else {
@@ -41,6 +74,19 @@ final class CatalogViewModel: ObservableObject {
             controller.boundaryState = .failed(message: "无法解析视频流地址")
             return
         }
+        if let document = boundaryDocuments[video.videoHash] {
+            let item = TrainingMediaItem(
+                id: video.videoHash,
+                title: video.title,
+                url: url,
+                sentences: document.sentences
+            )
+            controller.load(item)
+            controller.boundaryState = .loaded(count: document.sentences.count)
+            controller.showTransientMessage("课程已就绪：\(document.sentences.count) 句")
+            return
+        }
+
         let loadingItem = TrainingMediaItem(id: video.videoHash, title: video.title, url: url)
         controller.load(loadingItem)
         controller.boundaryState = .loading(isLocal: false)
@@ -51,6 +97,7 @@ final class CatalogViewModel: ObservableObject {
                 guard activeVideoHash == video.videoHash else {
                     return
                 }
+                boundaryDocuments[document.videoHash] = document
                 controller.updateBoundaries(document.sentences)
                 controller.boundaryState = .loaded(count: document.sentences.count)
                 controller.showTransientMessage("课程已就绪：\(document.sentences.count) 句")