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

README.md

@@ -199,7 +199,7 @@ The AppImage is all you need. First-run setup can install the optional `subminer
 
 Download the latest DMG from [GitHub Releases](https://github.com/ksyasuda/SubMiner/releases/latest) and drag `SubMiner.app` into `/Applications`.
 
-Then enable SubMiner under **System Settings > Privacy & Security > Accessibility**, or the overlay cannot follow the mpv window. If macOS blocks the first launch, right-click the app and choose **Open**.
+Then enable SubMiner under **System Settings > Privacy & Security > Accessibility** (named **Device Control and Data Access** on macOS 27), or the overlay cannot follow the mpv window. If macOS blocks the first launch, right-click the app and choose **Open**.
 
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