fix: firefox blank import validation screen and center toggles (#21266)

## Summary
Resolves #20182 and also centered toggles (e.g. under ICP) vertically in
validation cells.

## Screencasts
In Firefox:

Before:


https://github.com/user-attachments/assets/cd837733-6f4c-4bd4-9b08-723f22a5c9ba

After:


https://github.com/user-attachments/assets/d180a2f3-ee40-46f9-8db2-ecd341878fc6

## Screenshots
Before:
<img width="156" height="160" alt="Screenshot 2026-06-05 215835"
src="https://github.com/user-attachments/assets/a228fea8-7f3d-43f1-88bd-d6e198f8cac0"
/>

After:
<img width="191" height="148" alt="Screenshot 2026-06-05 215815"
src="https://github.com/user-attachments/assets/a419bf29-9925-44f2-a6c7-c11af6401806"
/>

---------

Signed-off-by: Parship Chowdhury <parshipchowdhury@gmail.com>
Co-authored-by: Charles Bochet <charles@twenty.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -24,6 +24,8 @@ const StyledDataGridContainer = styled.div<{ headerRowHeight?: number }>`
   --rdg-warning-cell-background-color: ${themeCssVariables.color.orange};
   --row-selected-hover-background-color: ${themeCssVariables.background
     .secondary};
+  flex: 1;
+  min-height: 0;
 
   > * {
     border: none;

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

@@ -184,6 +184,7 @@ export const generateColumns = (
                 }}
               >
                 <Toggle
+                  centered
                   value={row[columnKey] as boolean}
                   onChange={() => {
                     onRowChange({