fix(dashboard): prevent horizontal overflow when filter bar is open

The dashboard content grid item had the default `min-width: auto`,
which prevented it from shrinking below its content's intrinsic width.
When the vertical filter bar was open, this caused the content area to
overflow and produce an unwanted horizontal scrollbar. Adding
`min-width: 0` to StyledContent allows the grid item to properly shrink
within the `auto 1fr` grid layout.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

superset-frontend/src/dashboard/components/DashboardBuilder/DashboardBuilder.tsx

@@ -119,6 +119,7 @@ const StyledContent = styled.div<{
 }>`
   grid-column: 2;
   grid-row: 2;
+  min-width: 0;
   // @z-index-above-dashboard-header (100) + 1 = 101
   ${({ fullSizeChartId }) => fullSizeChartId && `z-index: 101;`}
 `;