refactor: Unify naming of direct/single/1:1/normal chats (#8442)

Unifies internal naming of single/direct/1:1/normal chats
to match the naming used in the API.

Closes: #8368
Signed-off-by: Jagoda Ślązak <jslazak@jslazak.com>

---------

Signed-off-by: Jagoda Ślązak <jslazak@jslazak.com>

src/calls.rs

@@ -201,7 +201,7 @@ impl Context {
         let chat = Chat::load_from_db(self, chat_id).await?;
         ensure!(
             chat.typ == Chattype::Single,
-            "Can only place calls in 1:1 chats"
+            "Can only place calls in single chats"
         );
         ensure!(!chat.is_self_talk(), "Cannot call self");