Fix item use (CMSG_USE_ITEM), mount tab, and inventory right-click

- Fix SMSG_ITEM_QUERY_SINGLE_RESPONSE parsing: read statsCount stat
  pairs instead of always 10, use 2 damage entries (MAX_ITEM_PROTO_DAMAGES),
  and parse item spell data (spellId + spellTrigger per slot)
- Pass item spell ID in CMSG_USE_ITEM packet so server processes item
  use requests (spellId=0 caused silent server rejection)
- Add spellId parameter to buildUseItem interface across all expansions
- Fix spellbook mount tab to use SkillLine 777 (Mounts) instead of 762
  (Riding), so known mount summon spells appear correctly
- Fix inventory right-click: use IsItemHovered+IsMouseClicked instead
  of IsItemClicked for InvisibleButton (which only tracks left-clicks)
- Fix SlotKind enum declaration order in inventory_screen.hpp

include/ui/inventory_screen.hpp

@@ -125,6 +125,19 @@ private:
     int heldBagSlotIndex = -1;
     game::EquipSlot heldEquipSlot = game::EquipSlot::NUM_SLOTS;
 
+    // Slot rendering with interaction support
+    enum class SlotKind { BACKPACK, EQUIPMENT };
+
+    // Click-and-hold pickup tracking
+    bool pickupPending_ = false;
+    float pickupPressTime_ = 0.0f;
+    SlotKind pickupSlotKind_ = SlotKind::BACKPACK;
+    int pickupBackpackIndex_ = -1;
+    int pickupBagIndex_ = -1;
+    int pickupBagSlotIndex_ = -1;
+    game::EquipSlot pickupEquipSlot_ = game::EquipSlot::NUM_SLOTS;
+    static constexpr float kPickupHoldThreshold = 0.12f; // seconds
+
     void renderSeparateBags(game::Inventory& inventory, uint64_t moneyCopper);
     void renderAggregateBags(game::Inventory& inventory, uint64_t moneyCopper);
     void renderBagWindow(const char* title, bool& isOpen, game::Inventory& inventory,
@@ -133,8 +146,6 @@ private:
     void renderBackpackPanel(game::Inventory& inventory, bool collapseEmptySections = false);
     void renderStatsPanel(game::Inventory& inventory, uint32_t playerLevel, int32_t serverArmor = 0);
 
-    // Slot rendering with interaction support
-    enum class SlotKind { BACKPACK, EQUIPMENT };
     void renderItemSlot(game::Inventory& inventory, const game::ItemSlot& slot,
                         float size, const char* label,
                         SlotKind kind, int backpackIndex,

include/ui/spellbook_screen.hpp

@@ -66,7 +66,7 @@ private:
     bool skillLineDbLoaded = false;
     std::unordered_map<uint32_t, std::string> skillLineNames;
     std::unordered_map<uint32_t, uint32_t> skillLineCategories;
-    std::unordered_map<uint32_t, uint32_t> spellToSkillLine;
+    std::unordered_multimap<uint32_t, uint32_t> spellToSkillLine;
 
     // Categorized spell tabs
     std::vector<SpellTabInfo> spellTabs;