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/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;