style: Replace inline border-top styles with reusable CSS class

Move border-top styling from inline attributes to .table-responsive--separator class in admin.css. Removes duplication and maintains consistent design system across all search+table pages (Blog, Clients, Inquiries).

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

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

@@ -29,7 +29,7 @@
         </div>
         @if (Model.Items.Count > 0)
         {
-            <div class="table-responsive" style="border-top: 1px solid var(--border-color-light);">
+            <div class="table-responsive table-responsive--separator">
                 <table class="table table-vcenter card-table table-hover">
                     <thead>
                         <tr>

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

@@ -42,7 +42,7 @@
         </div>
         @if (Model.Items.Count > 0)
         {
-            <div class="table-responsive" style="border-top: 1px solid var(--border-color-light);">
+            <div class="table-responsive table-responsive--separator">
                 <table class="table table-vcenter card-table table-hover">
                     <thead>
                         <tr>

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

@@ -20,7 +20,7 @@
         </div>
         @if (Model.Items.Count > 0)
         {
-            <div class="table-responsive" style="border-top: 1px solid var(--border-color-light);">
+            <div class="table-responsive table-responsive--separator">
                 <table class="table table-vcenter card-table table-hover">
                     <thead>
                         <tr>

src/TaxBaik.Web/wwwroot/css/admin.css

@@ -2118,3 +2118,11 @@ textarea:focus-visible {
   }
 }
 
+/* ============================================================================
+   Table Responsive Separator
+   ============================================================================ */
+
+.table-responsive--separator {
+  border-top: 1px solid var(--border-color-light);
+}
+