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/RestaurantList.tsx

@@ -2,18 +2,25 @@
 
 import type { Restaurant } from "@/lib/api";
 import { getCuisineIcon } from "@/lib/cuisine-icons";
+import { RestaurantListSkeleton } from "@/components/Skeleton";
 
 interface RestaurantListProps {
   restaurants: Restaurant[];
   selectedId?: string;
   onSelect: (r: Restaurant) => void;
+  loading?: boolean;
 }
 
 export default function RestaurantList({
   restaurants,
   selectedId,
   onSelect,
+  loading,
 }: RestaurantListProps) {
+  if (loading) {
+    return <RestaurantListSkeleton />;
+  }
+
   if (!restaurants.length) {
     return (
       <div className="p-4 text-center text-gray-500 text-sm">