Fix vendor buying, improve character select, parallelize WMO culling, and optimize collision

- Fix CMSG_BUY_ITEM count field from uint8 to uint32 (server silently dropped undersized packets)
- Character select screen: remember last selected character, two-column layout with details panel, double-click to enter world, responsive window sizing
- Fix stale character data between logins by replacing static init flag with per-character GUID tracking
- Parallelize WMO visibility culling across worker threads (same pattern as M2 renderer)
- Optimize WMO collision queries with world-space group bounds early rejection in getFloorHeight, checkWallCollision, isInsideWMO, and raycastBoundingBoxes
- Reduce camera ground samples from 5 to 3 movement-aligned probes
- Add WMO interior lighting, unlit materials, vertex color multiply, and alpha blending support

include/ui/character_screen.hpp

@@ -54,6 +54,7 @@ private:
     int selectedCharacterIndex = -1;
     bool characterSelected = false;
     uint64_t selectedCharacterGuid = 0;
+    bool restoredLastCharacter = false;
 
     // Status
     std::string statusMessage;
@@ -69,6 +70,13 @@ private:
      * Get faction color based on race
      */
     ImVec4 getFactionColor(game::Race race) const;
+
+    /**
+     * Persist / restore last selected character GUID
+     */
+    static std::string getConfigDir();
+    void saveLastCharacter(uint64_t guid);
+    uint64_t loadLastCharacter();
 };
 
 }} // namespace wowee::ui