feat: implement CMSG_PET_RENAME with rename dialog in pet frame

- Add PetRenamePacket::build(petGuid, name, isDeclined) builder
- Add GameHandler::renamePet(newName) — sends packet via petGuid_
- Add 'Rename Pet' to pet frame context menu (right-click pet name)
- Modal input dialog with 12-char limit matches server validation

include/game/game_handler.hpp

@@ -609,6 +609,7 @@ public:
     void cancelCast();
     void cancelAura(uint32_t spellId);
     void dismissPet();
+    void renamePet(const std::string& newName);
     bool hasPet() const { return petGuid_ != 0; }
     uint64_t getPetGuid() const { return petGuid_; }