MetroList: Make CSV Import Dialog Scrollable

Problem

CSV column mapping dialog had buttons that became inaccessible when importing files with many columns. The dialog didn't scroll, making the import impossible to complete.

Solution

Added vertical scroll to the column container in CsvColumnMappingDialog. Buttons are now always accessible regardless of column count.

Result

CSV imports work correctly for files with any number of columns.

Commit: MetrolistGroup/Metrolist@e8145dc
Issue: #2999