fix: Admin Home nav highlight stuck when visiting child routes

/admin/servers.startsWith('/admin') was true, so both Admin Home
and Server Fleet highlighted simultaneously. Now /admin uses
exact match like / does.

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

frontend/src/components/layout/DashboardLayout.vue

@@ -55,7 +55,7 @@ const adminNavItems = [
 ]
 
 function isActive(path: string): boolean {
-  if (path === '/') return route.path === '/'
+  if (path === '/' || path === '/admin') return route.path === path
   return route.path.startsWith(path)
 }