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

include/ui/game_screen.hpp

@@ -37,6 +37,9 @@ public:
      */
     bool isChatInputActive() const { return chatInputActive; }
 
+    void saveSettings();
+    void loadSettings();
+
 private:
     // Chat state
     char chatInputBuffer[512] = "";
@@ -66,10 +69,10 @@ private:
     int pendingSfxVolume = 100;
     float pendingMouseSensitivity = 0.2f;
     bool pendingInvertMouse = false;
-    int pendingUiOpacity = 100;
+    int pendingUiOpacity = 65;
 
     // UI element transparency (0.0 = fully transparent, 1.0 = fully opaque)
-    float uiOpacity_ = 1.0f;
+    float uiOpacity_ = 0.65f;
 
     /**
      * Render player info window
@@ -152,6 +155,7 @@ private:
     void renderWorldMap(game::GameHandler& gameHandler);
 
     InventoryScreen inventoryScreen;
+    uint64_t inventoryScreenCharGuid_ = 0;  // GUID of character inventory screen was initialized for
     QuestLogScreen questLogScreen;
     SpellbookScreen spellbookScreen;
     TalentScreen talentScreen;
@@ -174,6 +178,8 @@ private:
     int actionBarDragSlot_ = -1;
     GLuint actionBarDragIcon_ = 0;
 
+    static std::string getSettingsPath();
+
     // Left-click targeting: distinguish click from camera drag
     glm::vec2 leftClickPressPos_ = glm::vec2(0.0f);
     bool leftClickWasPress_ = false;