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/components/Feed.tsx

@@ -74,6 +74,7 @@ export default function Feed({
   view,
   canRead,
   isDemo = false,
+  plexEnabled = false,
   onOpenWizard,
   ytSearch,
   onYtSearch,
@@ -86,6 +87,8 @@ export default function Feed({
   view: "grid" | "list";
   canRead: boolean;
   isDemo?: boolean;
+  // When the optional Plex module is on, offer "Plex" as a feed source (App swaps to PlexBrowse).
+  plexEnabled?: boolean;
   onOpenWizard: () => void;
   // Live YouTube search: the active search term (null = normal feed). Entering a search and
   // leaving it both step browser history (the search is a feed sub-view with its own history
@@ -560,6 +563,7 @@ export default function Feed({
                 <option value="organic">{t("feed.source.organic")}</option>
                 <option value="all">{t("feed.source.all")}</option>
                 <option value="search">{t("feed.source.only")}</option>
+                {plexEnabled && <option value="plex">{t("feed.source.plex")}</option>}
               </select>
             </label>
           </>