refactor(ui): update theme colors for improved consistency and remove unused gradient background

lib/core/theme/app_theme.dart

@@ -116,8 +116,8 @@ class AppTheme {
     return baseTheme.copyWith(
       useMaterial3: true,
       colorScheme: AppColors.darkColorScheme,
-      scaffoldBackgroundColor: AppColors.background,
+      scaffoldBackgroundColor: const Color(0xFF1A1A1A),
-      canvasColor: AppColors.background,
+      canvasColor: const Color(0xFF1A1A1A),
 
       // ============ TYPOGRAPHY (Apple TV Optimized for 10ft Viewing) ============
       textTheme: TextTheme(

lib/main.dart

@@ -2,7 +2,6 @@ import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'core/router/app_router.dart';
 import 'core/theme/app_theme.dart';
-import 'core/theme/app_colors.dart';
 import 'core/providers/theme_provider.dart';
 
 void main() async {
@@ -25,25 +24,6 @@ class MyApp extends ConsumerWidget {
       darkTheme: AppTheme.darkTheme,
       themeMode: themeState.themeMode,
       routerConfig: router,
-      builder: (context, child) {
-        return Stack(
-          children: [
-            Container(
-              decoration: BoxDecoration(
-                gradient: AppColors.grayGlossyGradient,
-                boxShadow: [
-                  BoxShadow(
-                    color: Colors.white.withOpacity(0.08),
-                    blurRadius: 40,
-                    spreadRadius: 20,
-                  ),
-                ],
-              ),
-            ),
-            child!,
-          ],
-        );
-      },
     );
   }
 }