Add separate glass capsule for chapter title above seek preview

Extract chapter title from inside the storyboard preview into a
standalone ChapterCapsuleView with its own glass capsule background.
The capsule follows the seek position horizontally but independently
clamps to screen edges using alignmentGuide, allowing it to be wider
than the storyboard thumbnail without going offscreen.

Yattee/Views/Player/Gestures/GestureSeekPreviewView.swift

@@ -20,20 +20,31 @@ struct GestureSeekPreviewView: View {
     let theme: ControlsTheme
     let chapters: [VideoChapter]
     let isActive: Bool
+    var availableWidth: CGFloat = 320
 
     @State private var opacity: Double = 0
 
+    private var currentChapter: VideoChapter? {
+        chapters.last { $0.startTime <= seekTime }
+    }
+
     var body: some View {
-        Group {
-            // Only show if storyboard is available
+        VStack(spacing: 6) {
+            if let chapter = currentChapter {
+                ChapterCapsuleView(
+                    title: chapter.title,
+                    buttonBackground: buttonBackground
+                )
+                .frame(maxWidth: availableWidth - 16)
+            }
+
             if let storyboard {
                 SeekPreviewView(
                     storyboard: storyboard,
                     seekTime: seekTime,
                     storyboardService: storyboardService,
                     buttonBackground: buttonBackground,
-                    theme: theme,
-                    chapters: chapters
+                    theme: theme
                 )
             }
         }