feat: primary node switching in topology graph

public/scripts/api-client.js

@@ -72,6 +72,13 @@ class ApiClient {
         });
     }
 
+    async setPrimaryNode(ip) {
+        return this.request(`/api/discovery/primary/${encodeURIComponent(ip)}`, {
+            method: 'POST',
+            body: { timestamp: new Date().toISOString() }
+        });
+    }
+
     async getNodeStatus(ip) {
         return this.request(`/api/node/status/${encodeURIComponent(ip)}`, { method: 'GET' });
     }