feat(plex): field-scoped search (S2) + instant search typeahead

Add a search scope-pill to the Plex header search box (GitHub-style, one
field-scope at a time) and an instant typeahead of the top matches below it.

Backend (unified_library):
- New `field` param: all | title | people | summary | episode. `all` is
  byte-identical to the pre-scope behaviour (untouched weighted search_vector);
  a specific field keeps the GIN `search_vector @@ tsq` prefilter AND ANDs a
  per-column tsvector recheck, so scoped hits are a strict subset of Everywhere.
  Per-field expressions mirror the search_vector Computed def (same TS config,
  same left(summary,1000) truncation).
- `episode` scope empties the movie/show grid and title-only-matches the grouped
  Episodes section; the title/people/summary scopes suppress that section.
- Unknown field falls back to `all`. New DB-lane characterization tests
  (test_plex_field_scoped_search.py): each scope narrows correctly, field=all is
  identical to today, episode empties the grid and needs shows in scope.

Frontend:
- PlexSearchBox wraps the shared SearchBar with the scope-pill (reusing
  ActionMenu) and the typeahead; SearchBar gains generic leading/below/focus/
  keydown slots so other modules' search boxes are untouched.
- Episode scope offered only when shows are in scope (movie-only hides it),
  healed at the point of use like the sort state.
- Typeahead reuses /api/plex/library (limit=10), honours scope+field (not the
  sidebar filters), shows poster+title+year, keyboard nav (arrows/Enter/Esc),
  and jumps straight to a movie's info page or a show's drill-down via a new
  provider jumpTo hand-off (mirrors the playlistOpen pattern).
- `field` persisted per-account; en/hu labels added.

frontend/src/components/Header.tsx

@@ -5,11 +5,11 @@ import type { Me } from "../lib/api";
 import { moduleLabelKey, moduleOrder, stepModule } from "../lib/modules";
 import ModuleName from "./ModuleName";
 import SearchBar from "./SearchBar";
+import PlexSearchBox from "./PlexSearchBox";
 import SyncStatus from "./SyncStatus";
 import { useNavigation, useNavigationActions } from "./NavigationProvider";
 import { useFeedFilters, useFeedFiltersActions } from "./FeedFiltersProvider";
 import { usePlaylists, usePlaylistsActions } from "./PlaylistsProvider";
-import { usePlex, usePlexActions } from "./PlexProvider";
 import { useChannels, useChannelsActions } from "./ChannelsProvider";
 
 // The floating top header: a row of glass pills over the content — a ModuleName pill (label +
@@ -34,8 +34,6 @@ export default function Header({
   // "go to full history" jump is a ChannelsProvider action.
   const { search: playlistsSearch } = usePlaylists();
   const { setSearch: setPlaylistsSearch } = usePlaylistsActions();
-  const { q: plexQ } = usePlex();
-  const { setQ: setPlexQ } = usePlexActions();
   const { search: channelsSearch } = useChannels();
   const { setSearch: setChannelsSearch, goToFullHistory: onGoToFullHistory } = useChannelsActions();
   // The ◀/▶ arrows step cyclically through the user's available modules (derived from `me`, so
@@ -70,14 +68,7 @@ export default function Header({
       />
     );
   } else if (page === "plex" && me.plex_enabled) {
-    search = (
-      <SearchBar
-        value={plexQ}
-        placeholder={t("plex.searchPlaceholder")}
-        onChange={setPlexQ}
-        onGo={blur}
-      />
-    );
+    search = <PlexSearchBox />;
   } else if (page === "channels") {
     search = (
       <SearchBar