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

test/theme/app_dimens_test.dart

@@ -19,7 +19,7 @@ void main() {
     expect(AppDimens.sectionTitle, 16);
     expect(AppDimens.pageTitle, 20);
     expect(AppDimens.navLabel, 13);
-    expect(AppDimens.navLabelGap, 5);
+    expect(AppDimens.navLabelGap, 1);
     expect(AppDimens.navIcon, 30);
     expect(AppDimens.buttonHeight, 48);
     expect(AppDimens.primaryButtonHeight, 52);