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/api/plex.ts

@@ -4,6 +4,7 @@
 // touch only the shared `req`/`beacon` from core plus the local `appendPlexFilters` helper.
 import { req, beacon } from "./core";
 import { type SortState } from "../columnSort";
+import { type PlexSearchField } from "../plexSearch";
 
 // Admin: result of testing the Plex server connection (also drives the library-picker).
 interface PlexSection {
@@ -275,6 +276,7 @@ export const plexApi = {
   plexLibrary: (p: {
     scope: string; // movie | show | both
     q?: string;
+    field?: PlexSearchField; // search scope-pill (all | title | people | summary | episode)
     sort?: SortState;
     show?: string;
     offset?: number;
@@ -283,6 +285,7 @@ export const plexApi = {
   }): Promise<PlexUnifiedResult> => {
     const u = new URLSearchParams({ scope: p.scope });
     if (p.q) u.set("q", p.q);
+    if (p.field && p.field !== "all") u.set("field", p.field);
     if (p.sort) {
       u.set("sort", p.sort.key);
       u.set("sort_dir", p.sort.dir);