Improve login screen UI with simplified text and compatibility mode

- Change window title from "WoW 3.3.5a Authentication" to "Authentication"
- Change subtitle from "Connect to Authentication Server" to "Connect to Server"
- Increase button sizes from 120x0 to 160x40 for better visibility
- Add compatibility mode dropdown (currently showing 3.3.5a, extensible for future versions)

include/ui/auth_screen.hpp

@@ -46,6 +46,7 @@ private:
     char username[256] = "";
     char password[256] = "";
     int port = 3724;
+    int compatibilityMode = 0;  // 0 = 3.3.5a
     bool authenticating = false;
     bool showPassword = false;