feat: add Twig translation support for grid class labels

[denniserdmann]

Summary

• add translation support for grid_columns and grid_options via Symfony translation domain grid
• use translations/grid..yaml keys (group., column., option.*) when translated_labels: true
• keep fallback to raw CSS class names when a translation key is missing
• document the new translation approach in README

Why

Issue #8 asks for translations through Twig/Symfony translation files instead of contao/languages/*/default.php.

Notes

• Existing behavior remains unchanged unless translated_labels is enabled.