fix: add country flags to node usage display

- Add country_code to UserNodeUsageItem type
- Add getCountryFlag helper for country code to emoji mapping
- Show country flag next to node name in usage bars

src/api/adminUsers.ts

@@ -132,6 +132,7 @@ export interface UserPanelInfo {
 export interface UserNodeUsageItem {
   node_uuid: string;
   node_name: string;
+  country_code: string;
   total_bytes: number;
 }