feat(plex): P1.5 — Plex as a nav module + integrated search + filter sidebar

Per user feedback: promote Plex from a hidden feed-Source option to a first-class
left-nav module, with its own filter sidebar and the shared top search box.

- Plex is now a nav-rail module (page='plex', gated on me.plex_enabled) — its own
  page → correct browser Back/Forward; removed the Feed Source-dropdown 'plex' hack.
- PlexSidebar.tsx: left filter column (library scope, watch-state for movies:
  all/unwatched/in_progress/watched, sort) — per-account persisted (App state).
- Header search box now also serves the Plex page (integrated search); the live
  YouTube-search escalation stays feed-only.
- PlexBrowse.tsx reworked: infinite scroll (IntersectionObserver, no 'Load more'),
  content-visibility for smoother long-list scroll, show drill-down rides history
  (useHistorySubview) so Back returns to the grid; dropped the stray 'YouTube feed'
  button on the show page.
- backend /browse gains a per-user watch-state filter (show=, movies only).
- plex i18n (navLabel + filter.*) en/hu/de.

Note: episode-title 'Episode N' on some shows (e.g. Westworld) is Plex-side
(unmatched show) — we mirror what Plex has; Match/Refresh in Plex + re-sync fixes it.

frontend/src/components/Feed.tsx

@@ -74,7 +74,6 @@ export default function Feed({
   view,
   canRead,
   isDemo = false,
-  plexEnabled = false,
   onOpenWizard,
   ytSearch,
   onYtSearch,
@@ -87,8 +86,6 @@ 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
@@ -563,7 +560,6 @@ 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>
           </>