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

src/config/definitions/options-integrations.ts

@@ -554,7 +554,7 @@ export function buildIntegrationConfigOptionRegistry(
       kind: 'string',
       defaultValue: defaultConfig.mpv.executablePath,
       description:
-        'Optional absolute path to mpv.exe for Windows launch flows. Leave empty to auto-discover from SUBMINER_MPV_PATH or PATH.',
+        'Optional absolute path to the mpv binary SubMiner starts itself (Windows launches, Jellyfin casting, timing preview). Leave empty to auto-discover from SUBMINER_MPV_PATH, PATH, and on macOS the Homebrew and /Applications/mpv.app locations.',
     },
     {
       path: 'mpv.launchMode',