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.

backend/app/routes/me.py

@@ -12,6 +12,7 @@ from app.auth import (
     optional_current_user,
     purge_user,
 )
+from app import sysconfig
 from app.db import get_db
 from app.models import Invite, User
 
@@ -98,6 +99,8 @@ def get_me(
         # Instance-wide: whether Google sign-in / YouTube connect is available at all (the UI hides
         # YouTube-access affordances and the onboarding nudge when it isn't configured).
         "google_enabled": google_enabled(db),
+        # Whether the optional Plex module is enabled (UI shows the Plex feed source when so).
+        "plex_enabled": sysconfig.get_bool(db, "plex_enabled"),
         "can_read": has_read_scope(user),
         "can_write": has_write_scope(user),
         "pending_invites": pending_invites,