style: Compact admin UI density - reduce card padding and header size

- Reduce card-body padding from p-3 p-md-4 p-xl-4 to p-2 p-md-3
- Decrease admin-page-shell gap from 1rem to 0.5rem
- Reduce page header title size from 1.02rem to 0.9rem
- Reduce header margin-bottom from 0.5rem to 0.4rem
- Improve visual hierarchy with tighter spacing

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

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

@@ -114,7 +114,7 @@
                     </aside>
                     <main class="col-12 col-lg-10 col-xxl-10">
                         <div class="card shadow-sm border-0 admin-content-card admin-main-surface">
-                            <div class="card-body p-3 p-md-4 p-xl-4">
+                            <div class="card-body p-2 p-md-3">
                                 @RenderBody()
                             </div>
                         </div>

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

@@ -427,11 +427,12 @@ html, body {
 
 .admin-content-card .card-body {
   min-height: calc(100vh - 140px);
+  padding: 0.75rem 0.9rem !important;
 }
 
 .admin-page-shell {
   display: grid;
-  gap: 1rem;
+  gap: 0.5rem;
 }
 
 .details-grid dt {
@@ -491,17 +492,23 @@ html, body {
 }
 
 .admin-page-header-title {
-  font-size: 1.02rem;
-  line-height: 1.15;
+  font-size: 0.9rem;
+  line-height: 1.2;
   font-weight: 600;
+  margin-top: 0.15rem;
+  margin-bottom: 0.3rem;
 }
 
 .admin-page-header-subtitle {
-  font-size: 0.76rem;
+  font-size: 0.72rem;
+  line-height: 1.3;
+  margin-top: 0.2rem;
 }
 
 .admin-page-header-eyebrow {
-  font-size: 0.66rem;
+  font-size: 0.62rem;
+  margin-bottom: 0.2rem;
+  letter-spacing: 0.5px;
 }
 
 .breadcrumb-muted {
@@ -1276,21 +1283,25 @@ textarea:focus-visible {
 }
 
 .admin-page-shell .page-header {
-  margin-bottom: 0.5rem !important;
+  margin-bottom: 0.4rem !important;
 }
 
 .admin-page-header-eyebrow {
-  font-size: 0.68rem;
-  letter-spacing: 0.08em;
+  font-size: 0.62rem;
+  letter-spacing: 0.5px;
+  margin-bottom: 0.15rem;
 }
 
 .admin-page-header-title {
-  font-size: 1.08rem;
-  line-height: 1.15;
+  font-size: 0.9rem;
+  line-height: 1.2;
+  margin-top: 0;
+  margin-bottom: 0.25rem;
 }
 
 .admin-page-header-subtitle {
-  font-size: 0.78rem;
+  font-size: 0.72rem;
+  line-height: 1.3;
 }
 
 .admin-dashboard-header {