feat(plex): surface playback errors + media-mount status in the UI

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.

frontend/src/components/ConfigPanel.tsx

@@ -194,6 +194,14 @@ export default function ConfigPanel() {
                       version: plexResult.version ?? "",
                     })}
                   </p>
+                  {plexResult.media_check?.checked &&
+                    (plexResult.media_check.ok ? (
+                      <p className="text-[11px] text-emerald-500 mb-2">{t("config.plexMediaOk")}</p>
+                    ) : (
+                      <p className="text-[11px] text-red-400 mb-2">
+                        {t("config.plexMediaMissing", { path: plexResult.media_check.local_path ?? "" })}
+                      </p>
+                    ))}
                   <p className="text-[11px] text-muted mb-1.5">{t("config.plexPickLibraries")}</p>
                   <div className="flex flex-col gap-1.5">
                     {plexResult.sections.map((s) => (