Fix blog category fallback and empty card

src/TaxBaik.Application/Services/BlogService.cs

@@ -54,6 +54,7 @@ public class BlogService(
     public async Task<int> CreateAsync(BlogPost post, CancellationToken ct = default)
     {
         ValidatePost(post);
+        post.CategoryId ??= await EnsureCategoryIdAsync(0, ct);
         post.Title = post.Title.Trim();
         post.Content = post.Content.Trim();
         post.Slug = await GenerateUniqueSlugAsync(post.Title, ct: ct);
@@ -88,6 +89,7 @@ public class BlogService(
 
     public async Task UpdateAsync(BlogPost post, CancellationToken ct = default)
     {
+        post.CategoryId ??= await EnsureCategoryIdAsync(0, ct);
         await repository.UpdateAsync(post, ct);
         memoryCache.Remove(AdminDashboardService.CacheKey);
     }

src/TaxBaik.Web/Pages/Admin/Blog/Index.cshtml

@@ -25,16 +25,10 @@
 
     @if (Model.Items.Count == 0)
     {
-        <div class="empty py-5 border rounded bg-body">
-            <div class="empty-img">
-                <svg xmlns="http://www.w3.org/2000/svg" class="icon icon-tabler icon-tabler-database-off" width="48" height="48" viewBox="0 0 24 24" stroke-width="1.5" stroke="currentColor" fill="none" stroke-linecap="round" stroke-linejoin="round">
-                    <path d="M4 6c0 1.657 4.03 3 9 3s9-1.343 9-3-4.03-3-9-3-9 1.343-9 3"></path>
-                    <path d="M4 6v6c0 1.657 4.03 3 9 3s9-1.343 9-3V6"></path>
-                    <path d="M4 12v6c0 1.657 4.03 3 9 3s9-1.343 9-3v-6"></path>
-                    <path d="M3 3l18 18"></path>
-                </svg>
+        <div class="card">
+            <div class="card-body">
+                @await Html.PartialAsync("_EmptyState", "블로그 글이 없습니다.")
             </div>
-            <p class="empty-title">블로그 글이 없습니다.</p>
         </div>
     }
     else