feat(mornye): cache motion covers and refine player and search

Save album motion videos in app storage during downloads and prefer offline playback with an aspect-aware layout. Add cache management for saved videos.

Center active lyrics on opening, expand lyrics during manual scrolling, and adjust landscape header spacing. Group search rows and skeletons with dividers and remove redundant result icons.

Validation: Flutter analysis clean; 723 tests passed, 4 skipped; iOS simulator build installed.

lib/models/track.dart

@@ -14,6 +14,7 @@ class Track {
   final String? artistId;
   final String? albumId;
   final String? coverUrl;
+  final String? headerVideoUrl;
   final String? isrc;
   final String? previewUrl;
   final int duration;
@@ -46,6 +47,7 @@ class Track {
     this.artistId,
     this.albumId,
     this.coverUrl,
+    this.headerVideoUrl,
     this.isrc,
     this.previewUrl,
     required this.duration,
@@ -128,6 +130,7 @@ class Track {
       coverUrl: normalizeCoverReference(
         (data['cover_url'] ?? data['images'])?.toString(),
       ),
+      headerVideoUrl: normalizeRemoteHttpUrl(data['header_video']?.toString()),
       isrc: data['isrc']?.toString(),
       duration: (durationMs / 1000).round(),
       trackNumber: data['track_number'] as int?,
@@ -162,6 +165,7 @@ class Track {
     String? artistId,
     String? albumId,
     String? coverUrl,
+    String? headerVideoUrl,
     String? isrc,
     String? previewUrl,
     int? duration,
@@ -194,6 +198,7 @@ class Track {
       artistId: artistId ?? this.artistId,
       albumId: albumId ?? this.albumId,
       coverUrl: coverUrl ?? this.coverUrl,
+      headerVideoUrl: headerVideoUrl ?? this.headerVideoUrl,
       isrc: isrc ?? this.isrc,
       previewUrl: previewUrl ?? this.previewUrl,
       duration: duration ?? this.duration,