Add character creation screen with race/class/appearance customization

Implements a full character creation UI integrated into the existing flow.
In single-player mode, auth screen now goes to character creation before
entering the world. In online mode, a "Create Character" button on the
character selection screen sends CMSG_CHAR_CREATE to the server. Includes
WoW 3.3.5a race/class combo validation and appearance range limits.

include/core/application.hpp

@@ -20,6 +20,7 @@ namespace core {
 enum class AppState {
     AUTHENTICATION,
     REALM_SELECTION,
+    CHARACTER_CREATION,
     CHARACTER_SELECTION,
     IN_GAME,
     DISCONNECTED