improve: enhance combo components to production level (COMBO_POLICY compliance)

BlogForm:
- Add placeholder '分類 없음' for null category selection
- Label changed to '카테고리 (선택 사항)' to clarify null is allowed
- Add Clearable=true for easy null selection

InquiryForm:
- Add Required=true and Placeholder for ServiceType dropdown (mandatory field)
- Add Label asterisk (*) to indicate required field
- Add Clearable=true and Placeholder for Status dropdown (optional field)

Result: Combo components now follow COMBO_POLICY - null/required/optional states are
explicit in UI, not guessed by users. Aligns with 'Production Level' standard.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/TaxBaik.Web.Client/Components/Admin/Pages/Blog/BlogForm.razor

@@ -6,8 +6,9 @@
         <MudTextField @bind-Value="Model.Title" Label="제목 *"
             Variant="Variant.Outlined" Class="mb-4" Required="true" RequiredError="제목을 입력하세요." Counter="100" MaxLength="100" />
 
-        <MudSelect T="int?" @bind-Value="Model.CategoryId" Label="카테고리"
-            Variant="Variant.Outlined" Class="mb-4">
+        <MudSelect T="int?" @bind-Value="Model.CategoryId" Label="카테고리 (선택 사항)"
+            Variant="Variant.Outlined" Class="mb-4" Clearable="true">
+            <MudSelectItem Value="@((int?)null)">분류 없음</MudSelectItem>
             @foreach (var category in Categories)
             {
                 <MudSelectItem T="int?" Value="@((int?)category.Id)">@category.Name</MudSelectItem>