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/ui/character_create_screen.hpp

@@ -0,0 +1,42 @@
+#pragma once
+
+#include "game/character.hpp"
+#include "game/world_packets.hpp"
+#include <imgui.h>
+#include <string>
+#include <functional>
+#include <vector>
+
+namespace wowee {
+namespace game { class GameHandler; }
+
+namespace ui {
+
+class CharacterCreateScreen {
+public:
+    CharacterCreateScreen();
+
+    void render(game::GameHandler& gameHandler);
+    void setOnCreate(std::function<void(const game::CharCreateData&)> cb) { onCreate = std::move(cb); }
+    void setOnCancel(std::function<void()> cb) { onCancel = std::move(cb); }
+    void setStatus(const std::string& msg, bool isError = false);
+    void reset();
+
+private:
+    char nameBuffer[13] = {};  // WoW max name = 12 chars + null
+    int raceIndex = 0;
+    int classIndex = 0;
+    int genderIndex = 0;
+    int skin = 0, face = 0, hairStyle = 0, hairColor = 0, facialHair = 0;
+    std::string statusMessage;
+    bool statusIsError = false;
+
+    std::vector<game::Class> availableClasses;
+    void updateAvailableClasses();
+
+    std::function<void(const game::CharCreateData&)> onCreate;
+    std::function<void()> onCancel;
+};
+
+} // namespace ui
+} // namespace wowee