feat: restructure admin sidebar with collapsible sub-navigation and new routes

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/components/layout/AppSidebar.module.css

@@ -209,6 +209,44 @@
   text-align: center;
 }
 
+/* ─── Admin Sub-Menu ─── */
+.adminChevron {
+  margin-left: 6px;
+  font-size: 8px;
+  color: var(--text-muted);
+}
+
+.adminSubMenu {
+  display: flex;
+  flex-direction: column;
+}
+
+.adminSubItem {
+  display: block;
+  padding: 6px 16px 6px 42px;
+  font-size: 12px;
+  color: var(--text-muted);
+  text-decoration: none;
+  transition: all 0.1s;
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
+
+.adminSubItem:hover {
+  background: var(--bg-hover);
+  color: var(--text-primary);
+}
+
+.adminSubItemActive {
+  color: var(--amber);
+  background: var(--amber-glow);
+}
+
+.sidebarCollapsed .adminSubMenu {
+  display: none;
+}
+
 /* ─── Responsive ─── */
 @media (max-width: 1024px) {
   .sidebar {
@@ -242,4 +280,8 @@
   .sidebar .bottomLabel {
     display: none;
   }
+
+  .sidebar .adminSubMenu {
+    display: none;
+  }
 }