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

server/lib/websocket.ts

@@ -18,10 +18,12 @@ const nodes = new Map<string, WebSocket>();
 export function initWebSocketServer(wss: WebSocketServer) {
   wss.on("connection", (ws: WebSocket) => {
     let nodeId: string | null = null;
+    console.log("[WS] New connection");
 
     ws.on("message", async (raw) => {
       try {
         const msg: NodeMessage = JSON.parse(raw.toString());
+        console.log("[WS] Received:", msg.action, "from", msg.nodeId || nodeId);
 
         if (msg.action === "register" && msg.nodeId) {
           nodeId = msg.nodeId;
@@ -41,6 +43,7 @@ export function initWebSocketServer(wss: WebSocketServer) {
             where: { activationCode: msg.code },
           });
           if (!student) {
+            console.log("[WS] Invalid code:", msg.code);
             ws.send(JSON.stringify({ action: "activation_failed", error: "Invalid code" }));
             return;
           }
@@ -49,6 +52,7 @@ export function initWebSocketServer(wss: WebSocketServer) {
             update: { studentId: student.id, status: "online", lastSeen: new Date() },
             create: { id: nodeId, studentId: student.id, status: "online", lastSeen: new Date() },
           });
+          console.log("[WS] Activated:", student.firstName, student.lastName, "on", nodeId);
           ws.send(JSON.stringify({ action: "activated", studentId: student.id, studentName: `${student.firstName} ${student.lastName}` }));
           return;
         }
@@ -83,6 +87,7 @@ export function initWebSocketServer(wss: WebSocketServer) {
     });
 
     ws.on("close", async () => {
+      console.log("[WS] Connection closed for", nodeId);
       if (nodeId) {
         nodes.delete(nodeId);
         await prisma.node.upsert({