[Workflow] Add search in variable dropdown (#8479)

- fix Icon variable Plus
- add search input 
- fix dropdown height

## Before

![image](https://github.com/user-attachments/assets/49f73efd-21cc-4ecd-a494-f51edc34dc57)


## After

![image](https://github.com/user-attachments/assets/2af2c7ee-72fd-4dae-a1ef-19e75e1ac026)

packages/twenty-front/src/modules/ui/layout/dropdown/components/DropdownMenuHeader.tsx

@@ -15,9 +15,10 @@ const StyledHeader = styled.li`
   border-top-right-radius: ${({ theme }) => theme.border.radius.sm};
   border-bottom: 1px solid ${({ theme }) => theme.border.color.light};
 
-  padding: ${({ theme }) => theme.spacing(1)};
+  padding: ${({ theme }) => theme.spacing(1)} 0;
 
   user-select: none;
+  width: inherit;
 
   &:hover {
     background: ${({ theme, onClick }) =>