fix(jellyfin): find mpv when SubMiner is launched from Finder

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

docs-site/jellyfin-integration.md

@@ -25,7 +25,7 @@ SubMiner selects a Japanese subtitle track automatically and resets mpv's subtit
 
 If an episode has no Japanese subtitles, you can [generate them with Whisper](/subtitle-generation#generating-from-the-overlay) while it plays.
 
-On Windows, casting finds mpv through `mpv.executablePath`, then `SUBMINER_MPV_PATH`, then `PATH`. An invalid `mpv.executablePath` stops mpv from starting.
+Casting finds mpv through `mpv.executablePath`, then `SUBMINER_MPV_PATH`, then `PATH`. On macOS it then checks `/opt/homebrew/bin/mpv`, `/usr/local/bin/mpv`, and `/Applications/mpv.app`, because an app opened from Finder, the Dock, or Spotlight does not get your shell's `PATH`. If mpv lives somewhere else, set `mpv.executablePath` to its full path (`command -v mpv` prints it). An invalid `mpv.executablePath` stops mpv from starting.
 
 ## Playing from the terminal