- validera: role=tablist + role=tab + aria-selected + aria-controls/id
on tab buttons; role=tabpanel + id + aria-labelledby on content div
- katalog/[id]: same ARIA tab pattern; replaced incorrect aria-current=page
on tab buttons with role=tab + aria-selected
- error.tsx: i18n all hardcoded English strings via useT()
All three changes pass tsc 0.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>