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/i18n/locales/en/config.json

@@ -194,6 +194,8 @@
   "plexTestOk": "Connected to {{name}}",
   "plexTestFailed": "Plex connection failed — check the URL and token",
   "plexConnected": "Connected to {{name}} {{version}}",
+  "plexMediaOk": "Media mount OK — a sample file was found on disk.",
+  "plexMediaMissing": "Media not readable at {{path}} — check that the Plex media is bind-mounted into the container and that the path map is correct. Playback will fail until this is fixed.",
   "plexPickLibraries": "Libraries to expose:",
   "plexPickHint": "Unchecking all is the same as exposing every library. Remember to Save.",
   "plexMovies": "Movies",

frontend/src/i18n/locales/en/plex.json

@@ -47,7 +47,10 @@
     "tracks": "Audio & subtitles",
     "audio": "Audio",
     "subtitles": "Subtitles",
-    "subOff": "Off"
+    "subOff": "Off",
+    "errNotFound": "This file can't be played — the media isn't reachable on the server (the Plex media mount is missing or the path map is wrong). Ask the admin to check the Plex configuration.",
+    "errTranscode": "This file's format needs transcoding, which isn't supported yet.",
+    "errGeneric": "Playback couldn't start. Please try again."
   },
   "playable": {
     "direct": "Plays directly in the browser",