feat(plex): P1 frontend — Plex feed source, browse/search, show drill-down

- PlexBrowse.tsx: self-contained Plex mode (library scope tabs, own search +
  sort, poster-card grid with watch state + playability tint, paged; show →
  seasons/episodes drill-down). Playback announces (P2 wires the real player).
- Feed source dropdown gains a 'Plex' option (gated on me.plex_enabled); App
  swaps <Feed> for <PlexBrowse> when librarySource==='plex' so the video-feed
  hooks don't run in Plex mode. me exposes plex_enabled.
- api client (plexLibraries/plexBrowse/plexShow/plexImageUrl) + types; librarySource
  union + share-URL restore gain 'plex'; new plex i18n namespace en/hu/de.
- Verified over HTTP (authed): libraries, browse+FTS, image proxy 200 image/jpeg.

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

@@ -36,7 +36,8 @@
     "tip": "Filter by how videos got here: your subscriptions/catalog only, plus videos you found via live YouTube search, or only those search results.",
     "organic": "Hide search results",
     "all": "Include search results",
-    "only": "Search results only"
+    "only": "Search results only",
+    "plex": "Plex"
   },
   "yt": {
     "searchFor": "Search YouTube for “{{query}}”",