Cleanup: renaming "frame" to "character"

The "frame" variable actually contains a string pointing to the character selected.
It has nothing to do with a frame which is usually a particular image in an animation.

I'm renaming the variable accross the application to avoid confusion.

back/src/Controller/IoSocketController.ts

@@ -216,7 +216,7 @@ export class IoSocketController {
         socket.roomId = message.roomId;
         socket.userId = message.userId;
         socket.name = message.name;
-        socket.frame = message.frame;
+        socket.character = message.character;
     }
 
     refreshUserPosition() {