feat: mark merge request files as viewed in the sidebar

Closes #270

See merge request !3234

Merged-by: Elwyn Benson <ebenson@gitlab.com>
Approved-by: Elwyn Benson <ebenson@gitlab.com>
Reviewed-by: Elwyn Benson <ebenson@gitlab.com>
Reviewed-by: GitLab Duo <gitlab-duo@gitlab.com>
Co-authored-by: pavanatti <pavanatti@simplesdental.com>

# **pre-release**  [6.84.0](v6.83.4...v6.84.0) (2026-06-22)
### ✨ Features

* mark merge request files as viewed in the sidebar ([baa1c78a](baa1c78a)) by Henrique Pavanatti