feat(app): add vworld geocoding and native ads placeholders

lib/presentation/widgets/native_ad_placeholder.dart

@@ -0,0 +1,48 @@
+import 'package:flutter/material.dart';
+import 'package:lunchpick/core/constants/app_colors.dart';
+import 'package:lunchpick/core/constants/app_typography.dart';
+
+/// 네이티브 광고(Native Ad) 플레이스홀더
+class NativeAdPlaceholder extends StatelessWidget {
+  final EdgeInsetsGeometry? margin;
+  final double height;
+
+  const NativeAdPlaceholder({super.key, this.margin, this.height = 120});
+
+  @override
+  Widget build(BuildContext context) {
+    final isDark = Theme.of(context).brightness == Brightness.dark;
+
+    return Container(
+      margin: margin ?? EdgeInsets.zero,
+      padding: const EdgeInsets.all(16),
+      height: height,
+      width: double.infinity,
+      decoration: BoxDecoration(
+        color: isDark ? AppColors.darkSurface : Colors.white,
+        borderRadius: BorderRadius.circular(12),
+        border: Border.all(
+          color: isDark ? AppColors.darkDivider : AppColors.lightDivider,
+          width: 2,
+        ),
+        boxShadow: [
+          BoxShadow(
+            color: (isDark ? Colors.black : Colors.grey).withOpacity(0.08),
+            blurRadius: 8,
+            offset: const Offset(0, 4),
+          ),
+        ],
+      ),
+      child: Center(
+        child: Row(
+          mainAxisAlignment: MainAxisAlignment.center,
+          children: [
+            const Icon(Icons.ad_units, color: AppColors.lightPrimary, size: 24),
+            const SizedBox(width: 8),
+            Text('광고 영역', style: AppTypography.heading2(isDark)),
+          ],
+        ),
+      ),
+    );
+  }
+}