style: Extreme density optimization - ultra-compact admin UI

- Reduce admin-page-shell gap from 0.5rem to 0.25rem
- Reduce page header title from 0.9rem to 0.78rem
- Reduce header margins: eyebrow (0.2rem→0.1rem), subtitle (0.2rem→0.1rem), title (0.3rem→0.15rem)
- Reduce page-header margin-bottom from 0.4rem to 0.25rem
- Reduce card-body padding to p-2 across all breakpoints
- Reduce admin-page-shell padding from 1rem to 0.5rem
- Achieve Douzone-level data density

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-2 p-md-3">
+                            <div class="card-body p-2 p-md-2 p-lg-2">
                                 @RenderBody()
                             </div>
                         </div>

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

@@ -432,7 +432,7 @@ html, body {
 
 .admin-page-shell {
   display: grid;
-  gap: 0.5rem;
+  gap: 0.25rem;
 }
 
 .details-grid dt {
@@ -492,22 +492,22 @@ html, body {
 }
 
 .admin-page-header-title {
-  font-size: 0.9rem;
-  line-height: 1.2;
+  font-size: 0.78rem;
+  line-height: 1.1;
   font-weight: 600;
-  margin-top: 0.15rem;
-  margin-bottom: 0.3rem;
+  margin-top: 0 !important;
+  margin-bottom: 0.15rem !important;
 }
 
 .admin-page-header-subtitle {
-  font-size: 0.72rem;
-  line-height: 1.3;
-  margin-top: 0.2rem;
+  font-size: 0.7rem;
+  line-height: 1.25;
+  margin-top: 0.1rem;
 }
 
 .admin-page-header-eyebrow {
-  font-size: 0.62rem;
-  margin-bottom: 0.2rem;
+  font-size: 0.6rem;
+  margin-bottom: 0.1rem;
   letter-spacing: 0.5px;
 }
 
@@ -1278,12 +1278,17 @@ textarea:focus-visible {
    ============================================================================ */
 
 .admin-page-shell {
-  padding-top: 1rem;
-  padding-bottom: 1rem;
+  padding-top: 0.5rem;
+  padding-bottom: 0.5rem;
 }
 
 .admin-page-shell .page-header {
-  margin-bottom: 0.4rem !important;
+  margin-bottom: 0.25rem !important;
+}
+
+.admin-page-shell .page-header h2 {
+  margin-top: 0 !important;
+  margin-bottom: 0 !important;
 }
 
 .admin-page-header-eyebrow {