feat(Core UI): 白天黑夜模式

apps/im_app/lib/core/ui/base/colors.dart

@@ -41,17 +41,4 @@ class AppColors {
   // ── Neutral black Scale ─────────────────────────────────────────────────────
   static const black12 = Color(0x1F000000); // 12% opacity
   static const black60 = Color(0x99000000); // 60% opacity
-
-  /// 阴影颜色 Token
-  static Color shadow(BuildContext context) {
-    final brightness = Theme
-        .of(context)
-        .brightness;
-
-    if (brightness == Brightness.dark) {
-      return black60;
-    }
-
-    return black12;
-  }
 }

apps/im_app/lib/core/ui/base/shadows.dart

@@ -60,7 +60,7 @@ class AppShadows {
       BoxShadow(
 
         /// 阴影颜色来自 Design Token
-        color: AppColors.shadow(context),
+        color: _shadowColor,
 
         /// 模糊半径（影响阴影扩散范围）
         blurRadius: blur,
@@ -115,4 +115,15 @@ class AppShadows {
         blur: 16,
         dy: 8,
       );
+
+  /// 阴影颜色 Token
+  Color get _shadowColor {
+    final brightness = Theme
+        .of(context)
+        .brightness;
+
+    return brightness == Brightness.dark
+        ? AppColors.black60
+        : AppColors.black12;
+  }
 }