Add skeleton loading UI for better perceived performance

Replace "로딩 중..." text with animated skeleton placeholders in
RestaurantList, RestaurantDetail, and ReviewSection components.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/ReviewSection.tsx

@@ -200,7 +200,18 @@ export default function ReviewSection({ restaurantId }: ReviewSectionProps) {
       <h3 className="font-semibold text-sm mb-2">리뷰</h3>
 
       {loading ? (
-        <p className="text-sm text-gray-500">로딩 중...</p>
+        <div className="space-y-3 animate-pulse">
+          <div className="flex items-center gap-2">
+            <div className="h-4 w-24 bg-gray-200 rounded" />
+            <div className="h-4 w-8 bg-gray-200 rounded" />
+          </div>
+          {[1, 2].map((i) => (
+            <div key={i} className="space-y-1">
+              <div className="h-3 w-20 bg-gray-200 rounded" />
+              <div className="h-3 w-full bg-gray-200 rounded" />
+            </div>
+          ))}
+        </div>
       ) : (
         <>
           {reviewCount > 0 && avgRating !== null && (