fix(server): gestion erreur WebSocket au build + rewrite URL protocol-relative dans le proxy

- api/websocket: capture asynchrone EADDRINUSE pour ne pas bloquer le build
- proxy: réécrit aussi les URLs protocol-relatives (//localhost) dans le corps

server/app/api/websocket/route.ts

@@ -5,8 +5,14 @@ const globalWss = globalThis as typeof globalThis & { __eduboxWss?: WebSocketSer
 
 if (!globalWss.__eduboxWss) {
   try {
-    globalWss.__eduboxWss = new WebSocketServer({ port: 3001 });
-    initWebSocketServer(globalWss.__eduboxWss);
+    const wss = new WebSocketServer({ port: 3001 });
+    wss.on("error", (err) => {
+      // Silently ignore EADDRINUSE during build/hot reload; the existing
+      // server will keep handling agent connections.
+      console.warn("WebSocket server error:", (err as Error).message);
+    });
+    globalWss.__eduboxWss = wss;
+    initWebSocketServer(wss);
   } catch {
     // Port may be in use during build or hot reload
   }