style: Remove unnecessary spacing between page header and content

- Reduce admin-page-shell gap from 0.25rem to 0 (eliminate double spacing)
- Remove all card margins in admin-page-shell (margin-top/bottom: 0)
- Keep only page-header margin-bottom for section separation (0.5rem)
- Achieve seamless content flow with minimal visual breaks

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

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

@@ -432,7 +432,7 @@ html, body {
 
 .admin-page-shell {
   display: grid;
-  gap: 0.25rem;
+  gap: 0;
 }
 
 .details-grid dt {
@@ -451,6 +451,8 @@ html, body {
   border-radius: var(--radius-lg);
   box-shadow: none !important;
   border: 1px solid var(--border-color-light);
+  margin-top: 0 !important;
+  margin-bottom: 0 !important;
 }
 
 .blog-editor-shell .card-header,
@@ -1283,7 +1285,11 @@ textarea:focus-visible {
 }
 
 .admin-page-shell .page-header {
-  margin-bottom: 0.25rem !important;
+  margin-bottom: 0.5rem !important;
+}
+
+.admin-page-shell .card:first-of-type {
+  margin-top: 0 !important;
 }
 
 .admin-page-shell .page-header h2 {