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/lib/api.ts

@@ -992,12 +992,14 @@ export const api = {
     library: string;
     q?: string;
     sort?: string;
+    show?: string;
     offset?: number;
     limit?: number;
   }): Promise<PlexBrowseResult> => {
     const u = new URLSearchParams({ library: p.library });
     if (p.q) u.set("q", p.q);
     if (p.sort) u.set("sort", p.sort);
+    if (p.show && p.show !== "all") u.set("show", p.show);
     if (p.offset) u.set("offset", String(p.offset));
     if (p.limit) u.set("limit", String(p.limit));
     return req(`/api/plex/browse?${u.toString()}`);