Improve csv import (#5753)

This is a small PR to improve the design of our CSV import.

I noticed the back button that was implemented in a recent PR #5625 was
broken and would need to be fixed (e.g. try to come back to the very
first upload step from the sheet selection step). cc @shashankvish0010
if you want to give a stab at fixing your PR that'd be amazing, thanks!

packages/twenty-front/src/modules/ui/input/button/components/MainButton.tsx

@@ -3,13 +3,13 @@ import { useTheme } from '@emotion/react';
 import styled from '@emotion/styled';
 import { IconComponent } from 'twenty-ui';
 
-type Variant = 'primary' | 'secondary';
+export type MainButtonVariant = 'primary' | 'secondary';
 
 type Props = {
   title: string;
   fullWidth?: boolean;
   width?: number;
-  variant?: Variant;
+  variant?: MainButtonVariant;
   soon?: boolean;
 } & React.ComponentProps<'button'>;