feat: light background 및 color 추가 (#23)

Author: surfwithus

frontend/ongi/lib/core/app_colors.dart

@@ -3,4 +3,5 @@ import 'package:flutter/material.dart';
 class AppColors {
   static const ongiOrange = Color(0xFFFD6C01);
   static const ongiBlue = Color(0xFFBFECFF);
+  static const ongiLigntgrey = Color(0xFFF7F7F7);
 }

frontend/ongi/lib/core/app_light_background.dart

@@ -0,0 +1,31 @@
+import 'package:flutter/material.dart';
+import 'package:ongi/core/app_colors.dart';
+
+class AppLightBackground extends StatelessWidget {
+  final Widget child;
+  const AppLightBackground({super.key, required this.child});
+
+  @override
+  Widget build(BuildContext context) {
+    return Stack(
+      children: [
+        Container(
+          color: AppColors.ongiLigntgrey,
+        ),
+        Positioned(
+          right: -140,
+          top: -160,
+          child: Opacity(
+            opacity: 0.3,
+            child: Image.asset(
+              'assets/images/logo.png',
+              width: 350,
+              fit: BoxFit.contain,
+            ),
+          ),
+        ),
+        Positioned.fill(child: child),
+      ],
+    );
+  }
+}