style(nav): 对齐四 Tab 底栏布局规格

lib/core/theme/app_colors.dart

@@ -69,10 +69,9 @@ abstract final class AppColors {
   /// 底部导航底色。
   static const bar = ink;
 
-  /// 底部导航专用：纯黑背景、选中白、未选中灰与轻分割线。
+  /// 底部导航专用：纯黑背景、选中白与未选中灰。
   static const navBackground = Color(0xFF000000);
   static const navSelected = Color(0xFFFFFFFF);
-  static const navDivider = Color(0xFF1C1C1E);
 
   /// 卡片阴影（深色几乎不用阴影）。
   static const cardShadow = Color(0x00000000);
@@ -126,7 +125,7 @@ abstract final class AppColors {
   static const badgeBeige = raised;
 
   /// 导航未选中。
-  static const navUnselected = Color(0xFF8A8A8E);
+  static const navUnselected = Color(0xFF8E8E93);
 
   /// 播放器次要文字。
   static const playerMuted = Color(0xA3FFFFFF);

lib/core/theme/app_dimens.dart

@@ -37,6 +37,7 @@ abstract final class AppDimens {
   static const double previewSectionTitle = 14;
   static const double chipFont = 10;
   static const double navLabel = 11;
+  static const double navLabelGap = 4;
 
   static const double buttonHeight = 48;
   static const double primaryButtonHeight = 52;
@@ -47,8 +48,8 @@ abstract final class AppDimens {
   static const double tapTarget = 44;
   static const double contentHeaderHeight = 46;
 
-  static const double navHeight = 68;
-  static const double navIcon = 24;
+  static const double navHeight = 56;
+  static const double navIcon = 26;
   static const double progressHeight = 3;
   static const double progressRadius = 2;
   static const double progressLineHeight = 3;