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/lib/storage.ts

@@ -55,6 +55,7 @@ export const LS = {
   plexSort: "siftlode.plexSort",
   plexFilters: "siftlode.plexFilters",
   plexQ: "siftlode.plexQ",
+  plexField: "siftlode.plexField",
   plexPlaylistLayout: "siftlode.plexPlaylistLayout",
   plexPlayerPrefs: "siftlode.plexPlayerPrefs",
   // The YouTube modal player's volume (`{ volume: 0–100 }`; 0 IS muted — there is no separate