feat(cover): 支持横版封面并收紧底栏图标间距

- Drama 新增 coverLandscape，WatchProgress 新增 dramaCoverLandscape
- 继续观看横幅优先使用横版封面，缺省回退竖版
- 底部导航栏图标与文字间距由 5dp 收紧至 1dp

lib/core/data/models/drama.dart

@@ -97,6 +97,7 @@ class Drama {
     required this.title,
     required this.synopsis,
     required this.posterUrl,
+    this.coverLandscape = '',
     this.episodes = const [],
     this.titleLocales = const {},
     this.synopsisLocales = const {},
@@ -133,6 +134,9 @@ class Drama {
   final String tagCode;
   final String tagColor;
   final String posterUrl;
+
+  /// 横版封面，用于继续观看横幅等横向展示位；为空时回退竖版 `posterUrl`。
+  final String coverLandscape;
   final List<Episode> episodes;
   final String category;
   final Map<String, String> categoryLocales;
@@ -179,6 +183,7 @@ class Drama {
       tagCode: (json['tagCode'] as String? ?? '').trim(),
       tagColor: json['tagColor'] as String? ?? '',
       posterUrl: json['cover'] as String? ?? '',
+      coverLandscape: json['coverLandscape'] as String? ?? '',
       episodes: preview is Map<String, dynamic>
           ? [Episode.fromJson(preview)]
           : const [],
@@ -212,6 +217,7 @@ class Drama {
       tagCode: (json['tagCode'] as String? ?? '').trim(),
       tagColor: json['tagColor'] as String? ?? '',
       posterUrl: json['cover'] as String? ?? '',
+      coverLandscape: json['coverLandscape'] as String? ?? '',
       category: text.category,
       categoryLocales: text.categoryLocales,
       types: text.types,