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/ui/game_screen.hpp

@@ -434,6 +434,10 @@ private:
     char  gmTicketBuf_[2048] = {};
     void  renderGmTicketWindow(game::GameHandler& gameHandler);
 
+    // Pet rename modal (triggered from pet frame context menu)
+    bool petRenameOpen_ = false;
+    char petRenameBuf_[16] = {};
+
     // Inspect window
     bool  showInspectWindow_ = false;
     void  renderInspectWindow(game::GameHandler& gameHandler);