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/de/plex.json

@@ -0,0 +1,22 @@
+{
+  "backToFeed": "YouTube-Feed",
+  "backToLibrary": "Zurück zur Bibliothek",
+  "searchPlaceholder": "Plex durchsuchen…",
+  "sort": {
+    "added": "Kürzlich hinzugefügt",
+    "title": "Titel"
+  },
+  "count": "{{count}} Titel",
+  "searchCount": "{{count}} Treffer",
+  "loading": "Wird geladen…",
+  "empty": "Noch nichts hier. Starte eine Plex-Synchronisierung auf der Admin-Konfigurationsseite.",
+  "loadMore": "Mehr laden",
+  "watched": "Angesehen",
+  "seasons": "{{count}} Staffeln",
+  "playerSoon": "Player kommt bald — „{{title}}“",
+  "playable": {
+    "direct": "Spielt direkt im Browser",
+    "remux": "Braucht ein leichtes Remux (keine Video-Neucodierung)",
+    "transcode": "Braucht Transcoding (aufwendiger)"
+  }
+}