Add mobile menu for admin sidebar navigation

src/App.css

@@ -3725,6 +3725,27 @@
     color: rgba(240,234,216,0.45);
   }
 
+  .admin-mobile-menu-btn {
+    display: none;
+    background: transparent;
+    color: var(--brand-gold);
+    border: 1px solid rgba(201,168,76,0.35);
+    border-radius: 999px;
+    padding: 0.45rem 0.9rem;
+    font-family: var(--brand-font-body);
+    font-size: 0.8rem;
+    letter-spacing: 0.08em;
+    text-transform: uppercase;
+    cursor: pointer;
+    white-space: nowrap;
+  }
+
+  .admin-mobile-menu-btn--open {
+    background: rgba(201,168,76,0.15);
+    border-color: rgba(201,168,76,0.6);
+    color: var(--brand-warm-white);
+  }
+
   .admin-layout {
     display: flex;
     flex: 1;
@@ -3841,6 +3862,10 @@
     margin: 0;
   }
 
+  .admin-sidebar-backdrop {
+    display: none;
+  }
+
   .admin-panel-subhead {
     margin: 2rem 0 1rem;
     padding-bottom: 0.4rem;
@@ -4121,9 +4146,39 @@
 }
 
   @media (max-width: 768px) {
+    .admin-mobile-menu-btn {
+      display: inline-flex;
+      align-items: center;
+      justify-content: center;
+    }
+
     .admin-sidebar {
       display: none;
+      position: fixed;
+      top: 52px;
+      left: 0;
+      z-index: 130;
+      width: min(320px, 84vw);
+      min-width: 0;
+      height: calc(100vh - 52px);
+      border-right: 1px solid rgba(201,168,76,0.22);
+      box-shadow: 16px 0 40px rgba(0, 0, 0, 0.5);
     }
+
+    .admin-sidebar--open {
+      display: block;
+    }
+
+    .admin-sidebar-backdrop {
+      display: block;
+      position: fixed;
+      inset: 52px 0 0 0;
+      z-index: 120;
+      border: none;
+      background: rgba(0, 0, 0, 0.48);
+      cursor: pointer;
+    }
+
     .admin-panel {
       padding: 1.25rem 1rem;
     }