feat: replace gear dropdown with direct settings link

Remove the gear dropdown menu from AppHeader; cog icon is now a plain
router-link to /settings. Admin-only /admin/users and /admin/logs routes
are replaced with redirects to /settings. Mobile menu admin links removed.

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

frontend/src/router/index.ts

@@ -110,16 +110,8 @@ const router = createRouter({
       name: "settings",
       component: () => import("@/views/SettingsView.vue"),
     },
-    {
-      path: "/admin/users",
-      name: "admin-users",
-      component: () => import("@/views/UserManagementView.vue"),
-    },
-    {
-      path: "/admin/logs",
-      name: "admin-logs",
-      component: () => import("@/views/LogsView.vue"),
-    },
+    { path: "/admin/users", redirect: "/settings" },
+    { path: "/admin/logs", redirect: "/settings" },
   ],
 });