refactor: consolidate buildItemLink into game_utils.hpp

Three identical copies (game_handler.cpp, spell_handler.cpp,
quest_handler.cpp) plus two forward declarations (inventory_handler.cpp,
social_handler.cpp) replaced with a single inline definition in
game_utils.hpp. All affected files already include this header, so
quality color table changes now propagate from one source of truth.

src/game/social_handler.cpp

@@ -17,8 +17,6 @@
 namespace wowee {
 namespace game {
 
-// Free function defined in game_handler.cpp
-std::string buildItemLink(uint32_t itemId, uint32_t quality, const std::string& name);
 
 static bool packetHasRemaining(const network::Packet& packet, size_t need) {
     const size_t size = packet.getSize();