Prepare release 0.8.1

- Update mpv to 0.39.0
  -> mpv-dev-x86_64-20241027-git-fbf8695
- Update python-mpv to 1.0.7
  -> e1ae4f7da6d1858ab5af7d003c24fd6ca1f8fed5
- Update Justfile
- Remove obsolete credits

Prepare Migration to Upstream Project (#144)

* Checkout translations from upstream
* Update translations
* Update flatpak appstream file
* Update documentation
* Run full release build on main branch only

Regenerate resources

Update README.md

add files