feat: add AchievementsSkeleton and improve dashboard loading state

Author: Heeesang

app/(root)/dashboard/loading.tsx

@@ -1,12 +1,23 @@
 import StatsCardSkeleton from '@/components/dashboard/StatsCardSkeleton';
+import AchievementsSkeleton from '@/components/dashboard/AchievementsSkeleton';
 
 export default function DashboardLoading() {
   return (
     <div className="p-4 md:p-6 lg:p-8 min-h-screen">
       <div className="grid grid-cols-1 lg:grid-cols-[300px_1fr_320px] gap-6 lg:gap-8">
         {/* Left Sidebar Skeleton */}
         <div className="flex flex-col gap-6">
-          <div className="h-[480px] rounded-2xl shimmer border border-white/10" />
+          {/* Profile Card Skeleton Placeholder */}
+          <div className="h-100 rounded-2xl shimmer border border-white/10" />
+
+          {/* Achievements Skeleton */}
+          <div className="p-6 rounded-xl bg-[#0a0a0a] border border-[rgba(255,255,255,0.08)]">
+            <div className="flex items-center gap-2.5 mb-5">
+              <div className="w-4 h-4 shimmer rounded" />
+              <div className="w-24 h-4 shimmer rounded" />
+            </div>
+            <AchievementsSkeleton />
+          </div>
         </div>
 
         {/* Main Content Skeleton */}

components/dashboard/AchievementsSkeleton.tsx

@@ -0,0 +1,9 @@
+export default function AchievementsSkeleton() {
+  return (
+    <div className="grid grid-cols-2 gap-2">
+      {[...Array(4)].map((_, i) => (
+        <div key={i} className="h-16 shimmer rounded border border-white/5" />
+      ))}
+    </div>
+  );
+}