PlexPlayer: when the stream session can't start, show why instead of an eternal
spinner — 404 (physical file unreachable: missing media mount / wrong path map),
501 (format needs transcoding), or a generic failure; also catch fatal hls.js
errors. ConfigPanel: render the Test-connection media_check (mount OK, or a
warning naming the unreadable local path). New plex/config i18n in en/hu/de.