Align admin home with Tabler tone

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

@@ -20,7 +20,7 @@
             <div class="card card-sm">
                 <div class="card-body">
                     <div class="text-muted small text-uppercase mb-1">진입</div>
-                    <h3 class="h4 mb-2">대시보드</h3>
+                    <h3 class="h5 mb-2">대시보드</h3>
                     <p class="text-muted mb-0">업무 시작점과 주요 상태를 확인합니다.</p>
                 </div>
                 <div class="card-footer">
@@ -32,7 +32,7 @@
             <div class="card card-sm">
                 <div class="card-body">
                     <div class="text-muted small text-uppercase mb-1">세션</div>
-                    <h3 class="h4 mb-2">인증 상태</h3>
+                    <h3 class="h5 mb-2">인증 상태</h3>
                     <p class="text-muted mb-0">쿠키 인증과 배포 간 유지성을 확인합니다.</p>
                 </div>
                 <div class="card-footer">
@@ -44,7 +44,7 @@
             <div class="card card-sm">
                 <div class="card-body">
                     <div class="text-muted small text-uppercase mb-1">구조</div>
-                    <h3 class="h4 mb-2">Tabler UI</h3>
+                    <h3 class="h5 mb-2">Tabler UI</h3>
                     <p class="text-muted mb-0">공통 카드, 테이블, 배지, 폼을 표준화합니다.</p>
                 </div>
                 <div class="card-footer">

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

@@ -346,10 +346,7 @@ html, body {
 }
 
 .admin-shell {
-  background:
-    radial-gradient(circle at top left, rgba(37, 99, 235, 0.08), transparent 26%),
-    radial-gradient(circle at top right, rgba(15, 23, 42, 0.04), transparent 22%),
-    linear-gradient(180deg, #F5F8FC 0%, #EDF2F8 100%);
+  background: #f6f8fb;
 }
 
 .admin-topbar {
@@ -366,7 +363,7 @@ html, body {
 }
 
 .admin-sidebar-header {
-  background: linear-gradient(180deg, #FFFFFF 0%, #F8FAFC 100%);
+  background: #ffffff;
   border-bottom: 1px solid var(--border-color-light);
   padding-top: 0.5rem;
   padding-bottom: 0.5rem;
@@ -387,7 +384,7 @@ html, body {
 
 .admin-sidebar-card .accordion-button:not(.collapsed) {
   color: var(--primary-dark);
-  background: linear-gradient(90deg, rgba(31, 78, 121, 0.08), rgba(31, 78, 121, 0.03));
+  background: #f7f9fc;
   box-shadow: inset 3px 0 0 var(--primary-color);
 }
 
@@ -406,7 +403,7 @@ html, body {
 }
 
 .admin-sidebar-card .list-group-item.active {
-  background: linear-gradient(90deg, rgba(31, 78, 121, 0.14), rgba(31, 78, 121, 0.06));
+  background: #eef4fb;
   color: var(--primary-dark);
   font-weight: var(--font-weight-semibold);
 }
@@ -1334,6 +1331,7 @@ textarea:focus-visible {
 
 .admin-dashboard-metrics .card {
   min-height: 100%;
+  background: #ffffff;
 }
 
 .blog-editor-header .badge-sm {