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/storage.ts

@@ -23,6 +23,9 @@ export const LS = {
   filterCollapsed: "siftlode.filterCollapsed",
   playlist: "siftlode.playlist",
   plSort: "siftlode.plSort",
+  plexLibrary: "siftlode.plexLibrary",
+  plexShow: "siftlode.plexShow",
+  plexSort: "siftlode.plexSort",
   notifHistory: "siftlode.notifications",
   notifSettings: "siftlode.notifSettings",
   onboardingDismissed: "siftlode.onboarding.dismissed",