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/plex.json

@@ -0,0 +1,22 @@
+{
+  "backToFeed": "YouTube feed",
+  "backToLibrary": "Back to library",
+  "searchPlaceholder": "Search Plex…",
+  "sort": {
+    "added": "Recently added",
+    "title": "Title"
+  },
+  "count": "{{count}} titles",
+  "searchCount": "{{count}} matches",
+  "loading": "Loading…",
+  "empty": "Nothing here yet. Run a Plex sync from the admin Config page.",
+  "loadMore": "Load more",
+  "watched": "Watched",
+  "seasons": "{{count}} seasons",
+  "playerSoon": "Player coming soon — “{{title}}”",
+  "playable": {
+    "direct": "Plays directly in the browser",
+    "remux": "Needs a light remux (no video re-encode)",
+    "transcode": "Needs transcoding (heavier)"
+  }
+}