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

changes/macos-mpv-discovery-docs.md

@@ -0,0 +1,4 @@
+type: docs
+area: docs
+
+- Documented how casting finds mpv on every platform, added a troubleshooting entry for casts that never start mpv, and noted that macOS 27 renames the Accessibility privacy list to "Device Control and Data Access".