Fix broken CSV import grid layout (#21867)

## What

Import `react-data-grid/lib/styles.css` in `SpreadsheetImportTable`, the
single component that renders the import grid (used by the Validate Data
and Select Header steps).

## Why

The React 19 migration (#21531) bumped `react-data-grid` from
`7.0.0-beta.13` to `7.0.0-beta.59`. The old beta auto-injected its
layout CSS; beta.59 ships it as a separate
`react-data-grid/lib/styles.css` export that must be imported manually.
It was never imported, so the grid lost its base layout (grid template,
row heights, cell positioning): rows stacked at full height and columns
no longer aligned. The library scopes its styles under `@layer rdg`, so
the existing Linaria theme overrides still take precedence.

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## Before
<img width="2540" height="1448" alt="CleanShot 2026-06-19 at 17 43
58@2x"
src="https://github.com/user-attachments/assets/a208b518-9088-4988-8245-4fdc4f8bc8de"
/>


## After
<img width="2454" height="1392" alt="CleanShot 2026-06-19 at 18 02
36@2x"
src="https://github.com/user-attachments/assets/e0b30d71-8244-4363-86aa-60b12a2dfdd9"
/>

packages/twenty-front/src/modules/spreadsheet-import/components/SpreadsheetImportTable.tsx

@@ -1,6 +1,7 @@
 import { styled } from '@linaria/react';
 import { useContext } from 'react';
 import { DataGrid, type DataGridProps } from 'react-data-grid';
+import 'react-data-grid/lib/styles.css';
 import { useSpreadsheetImportInternal } from '@/spreadsheet-import/hooks/useSpreadsheetImportInternal';
 import { ThemeContext, themeCssVariables } from 'twenty-ui/theme-constants';
 

packages/twenty-front/src/modules/spreadsheet-import/steps/components/ValidationStep/components/columns.tsx

@@ -56,10 +56,12 @@ const StyledInputContainer = styled.div`
 `;
 
 const StyledDefaultContainer = styled.div`
+  align-content: center;
   min-height: 100%;
   min-width: 100%;
   overflow: hidden;
   text-overflow: ellipsis;
+  white-space: nowrap;
 `;
 
 const StyledSelectReadonlyValueContianer = styled.div`