Phase 4.5: Completed responsive player layout and real username integration

backend/internal/api/auth.go

@@ -91,6 +91,7 @@ func Login(c *gin.Context) {
 	}
 
 	c.JSON(http.StatusOK, gin.H{
-		"token": token,
+		"token":    token,
+		"username": user.Username,
 	})
 }