fix(jellyfin): respect Windows mpv configuration when casting (#267)

changes/jellyfin-configured-mpv-docs.md

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+type: docs
+area: jellyfin
+
+- Clarify that Windows mpv playback and Jellyfin casting can use a configured executable path instead of PATH.

changes/jellyfin-configured-mpv.md

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+type: fixed
+area: jellyfin
+
+- Honor the configured mpv executable when Jellyfin starts playback, allowing casting when mpv is installed outside PATH, and detect portable plugins beside the selected executable.