fix: drawer responsiveness on mobile (375px)

Mobile S (<480px) drawer now properly:
- Uses flex-direction: row for horizontal layout
- Has max-height: 60px to constrain vertical space
- Shows horizontal scrollbar for nav items (overflow-x: auto)
- Proper border styling (no right border, bottom border)
- Brand mark positioned correctly with flex-shrink: 0

This fixes the drawer responsiveness test on 375px viewport.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

TaxBaik.Web/wwwroot/css/admin.css

@@ -1201,11 +1201,19 @@ textarea:focus-visible {
   .admin-drawer {
     width: 100%;
     height: auto;
+    max-height: 60px;
+    flex-direction: row;
+    border-right: none;
+    border-bottom: 1px solid var(--border-color);
+    overflow-x: auto;
   }
 
   .admin-drawer-brand {
     padding: var(--space-3) var(--space-4);
     gap: var(--space-2);
+    flex-shrink: 0;
+    border-bottom: none;
+    border-right: 1px solid var(--border-color-light);
   }
 
   .admin-brand-mark {