refactor(auth): centralize role checks into utility

Removed direct user role string checks and replaced with type-safe utils.

app/components/Header.vue

@@ -34,7 +34,7 @@ const navLinks = [
 ]
 
 const { isAuthenticated, user, logout } = useAuth()
-const panelLabel = computed(() => user.value?.role?.toUpperCase() === 'ADMIN' ? 'Pannello' : 'Dashboard')
+const panelLabel = computed(() => isAdmin(user.value?.role) ? 'Pannello' : 'Dashboard')
 </script>
 
 <template>