style: Update header actions layout with improved button grouping and dropdown z-index for better overlay on tables/cards

frontend/src/App.css

@@ -239,6 +239,8 @@
   align-items: center;
   gap: 0.5rem;
 }
+.header-actions .btn-group .btn-group { gap: 0; }
+.header-actions .btn-group .dropdown-toggle-split { padding-left: .5rem; padding-right: .5rem; }
 .header-actions .btn {
   display: inline-flex;
   align-items: center;
@@ -246,6 +248,12 @@
 }
 .header-actions .btn svg { width: 18px; height: 18px; }
 
+/* Ensure dropdowns in header-actions overlay tables/cards correctly */
+.header-actions .dropdown-menu,
+.page-header .dropdown-menu {
+  z-index: 2000;
+}
+
 /* Responsive sizing for small screens */
 @media (max-width: 992px) {
   .header-actions { gap: 0.4rem; }