add: ability to add user to stack through block list ui

crates/app/src/routes/api/v1/stacks.rs

@@ -211,6 +211,10 @@ pub async fn add_user_request(
         Err(e) => return Json(e.into()),
     };
 
+    if stack.users.contains(&other_user.id) {
+        return Json(Error::MiscError("This user is already in this stack".to_string()).into());
+    }
+
     stack.users.push(other_user.id);
 
     // check number of stacks