fix(ui): translate DiffCollapser expand/collapse aria-label

[jhjh1214]

What?

The expand/collapse toggle button in DiffCollapser used hardcoded English strings 'Expand' and 'Collapse' for its aria-label. The translation keys general:expand and general:collapse already exist, and useTranslation was already imported and t already in scope — the strings just weren't being used.

How?

aria-label={isCollapsed ? t('general:expand') : t('general:collapse')}