fix: activation via connexion WS principale, cascade delete student→nodes, lien fiche étudiant

- agent/websocket.go: expose sendMessage() + notifyUI() pour broadcaster
  les résultats d'activation à tous les clients UI connectés
- agent/ui.go: supprime forwardActivation(), utilise sendMessage() sur
  la connexion WS principale au lieu d'une connexion temporaire
- agent/activation.go: ajoute os.MkdirAll avant l'écriture d'activation.json
- server/prisma/schema.prisma: onDelete Cascade sur Node→Student
- server/app/dashboard/students/page.tsx: nom cliquable vers fiche détail
- server/app/dashboard/students/[id]/actions.ts: deleteMany → delete

agent/ui.go

@@ -29,13 +29,27 @@ func startUI(dataDir, nodeID, serverAddr string) {
 			return
 		}
 		defer conn.Close()
+		log.Printf("UI client connected from %s", r.RemoteAddr)
+
+		// Register notifier to forward activation results from main WS to this UI connection
+		notifierID := registerUINotifier(func(msg map[string]interface{}) {
+			log.Printf("UI notifier forwarding to browser: %+v", msg)
+			if err := conn.WriteJSON(msg); err != nil {
+				log.Printf("UI notify error: %v", err)
+			} else {
+				log.Printf("UI notifier sent successfully")
+			}
+		})
+		defer unregisterUINotifier(notifierID)
 
 		for {
 			var msg map[string]interface{}
 			if err := conn.ReadJSON(&msg); err != nil {
+				log.Printf("UI client disconnected: %v", err)
 				break
 			}
 			action, _ := msg["action"].(string)
+			log.Printf("UI received action: %s", action)
 			switch action {
 			case "check":
 				act, err := loadActivation(dataDir)
@@ -46,10 +60,13 @@ func startUI(dataDir, nodeID, serverAddr string) {
 				}
 			case "activate":
 				code, _ := msg["code"].(string)
-				// Forward to server WS
-				go func() {
-					forwardActivation(serverAddr, nodeID, code, conn)
-				}()
+				log.Printf("UI handling activate with code: %s", code)
+				if err := sendMessage(WSMessage{Action: "activate", NodeID: nodeID, Code: code}); err != nil {
+					log.Printf("UI sendMessage failed: %v", err)
+					conn.WriteJSON(map[string]interface{}{"action": "activation_failed", "error": err.Error()})
+				} else {
+					log.Printf("UI sendMessage succeeded, waiting for server response...")
+				}
 			case "instances":
 				listInstances(dataDir, conn)
 			}
@@ -63,30 +80,6 @@ func startUI(dataDir, nodeID, serverAddr string) {
 	}
 }
 
-func forwardActivation(serverAddr, nodeID, code string, uiConn *websocket.Conn) {
-	conn, _, err := websocket.DefaultDialer.Dial(serverAddr, nil)
-	if err != nil {
-		uiConn.WriteJSON(map[string]interface{}{"action": "activation_failed", "error": err.Error()})
-		return
-	}
-	defer conn.Close()
-
-	conn.WriteJSON(map[string]interface{}{"action": "register", "nodeId": nodeID})
-	conn.WriteJSON(map[string]interface{}{"action": "activate", "code": code, "nodeId": nodeID})
-
-	for {
-		var msg map[string]interface{}
-		if err := conn.ReadJSON(&msg); err != nil {
-			break
-		}
-		action, _ := msg["action"].(string)
-		if action == "activated" || action == "activation_failed" {
-			uiConn.WriteJSON(msg)
-			break
-		}
-	}
-}
-
 func listInstances(dataDir string, conn *websocket.Conn) {
 	dir := filepath.Join(dataDir, "instances")
 	entries, err := os.ReadDir(dir)