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/plexSearch.ts

@@ -0,0 +1,34 @@
+// The search-box scope-pill (S2): one field-scope active at a time, narrowing the Plex full-text
+// search to a single facet of the row. Kept in one module so the provider (state), the header
+// SearchBar (the pill UI), and the api layer (the `field` query param) share one source of truth.
+
+export type PlexSearchField = "all" | "title" | "people" | "summary" | "episode";
+
+export const PLEX_SEARCH_FIELDS: PlexSearchField[] = [
+  "all",
+  "title",
+  "people",
+  "summary",
+  "episode",
+];
+
+// i18n label key per scope (see plex.searchField.* in the locale files).
+export const plexFieldLabelKey = (f: PlexSearchField): string => `plex.searchField.${f}`;
+
+// Episodes only exist under show libraries, so the Episode scope is offered only when shows are in
+// scope (show | both) — mirroring the backend, which returns nothing for episode+movie-only.
+export function availablePlexFields(scope: string): PlexSearchField[] {
+  return scope === "movie"
+    ? PLEX_SEARCH_FIELDS.filter((f) => f !== "episode")
+    : PLEX_SEARCH_FIELDS;
+}
+
+// Heal a stored/selected field against the current scope: a movie-only scope can't hold Episode, so
+// it falls back to Everywhere (analogous to healPlexSort). Applied at the point of use.
+export function healPlexField(field: PlexSearchField, scope: string): PlexSearchField {
+  return availablePlexFields(scope).includes(field) ? field : "all";
+}
+
+export function parsePlexField(v: string | null | undefined): PlexSearchField {
+  return v && (PLEX_SEARCH_FIELDS as string[]).includes(v) ? (v as PlexSearchField) : "all";
+}