fix: detect system-wide flatpak mpv installs

[port19x]

Pull Request Template

Type of change

• Bug fix
• Feature
• Documentation update

Description

First time using Codex. And AI outside the browser for that matter.

fixes: #1849

Checklist

• any anime playing
• bumped version

---

• next, prev, replay and select work
• -c history and continue work
• -d downloads work
• -e (select episode) aka -r (range selection) works
• -S select index works
• -q quality works
• -s syncplay works
• -v vlc works
• --dub and regular (sub) mode both work
• --nextep-countdown countdown to next ep works

---

• -h help info is up to date
• Readme is up to date
• Man page is up to date

Additional Testcases

(these don't block a merge)

• --skip ani-skip works
• --no-detach no detach works
• --exit-after-play auto exit after playing works