Apps opened from Finder, the Dock, or Spotlight inherit launchd's minimal
PATH, so spawning bare `mpv` failed for Homebrew installs and Jellyfin
casting never got an IPC socket.
- On macOS and Linux, resolve mpv from mpv.executablePath,
SUBMINER_MPV_PATH, PATH, then (macOS) Homebrew, /usr/local/bin, and
/Applications/mpv.app
- An invalid mpv.executablePath errors instead of silently falling back
- Timing preview uses the same lookup
- Docs: mpv lookup order, a Jellyfin casting troubleshooting entry, and
the macOS 27 rename of Accessibility to Device Control and Data Access
Closes#297
