E5 S7: Plex filter UI — listbox-select pickers + studio filter

Reworks the always-expanded filter chip walls into compact, Seerr-style
"+ Add" → searchable-popover pickers (the user's space-saving ask), and adds
the missing studio filter.

- New MultiSelectChips: selected values as removable chips + a "+ Add" trigger
  opening a searchable checklist (AnchoredPopover platform — menuitemcheckbox,
  arrow/Escape, anchor flip). Options carry facet counts; a large vocabulary
  renders a 50-row cap + "N more — refine with search" footer (studios run to
  ~1500). Replaces the flat Genre + Age-rating chip lists.
- New Studio filter via the same picker. Backend: /facets now aggregates the
  studio vocabulary (distinct studio + count, self-excluded like genres). The
  studio filter itself already existed (only addable from the detail page before).
- Collection converted to the same "+ Add" pattern (CollectionPicker): a
  single-select popover with server-side search + the 50-cap; the selected
  collection still shows as a removable chip. Retires the always-inline search+list.

New i18n plex.filter.studio/add/moreRefine + common.search/remove/noResults
(en+hu). Tests: MultiSelectChips.dom.test.tsx (5, incl. the cap) +
test_plex_facets studio-vocabulary. Browser-E2E'd: Genre→Horror(402),
Studio→A24(14, capped/searched), Collection→James Bond(26)/harry-search.
Gate: vitest 342, backend lane 403.

frontend/src/lib/api/plex.ts

@@ -288,6 +288,7 @@ function appendPlexFilters(u: URLSearchParams, f: PlexFilters): void {
 export interface PlexFacets {
   genres: { value: string; count: number }[];
   content_ratings: { value: string; count: number }[];
+  studios: { value: string; count: number }[]; // studio vocabulary for the sidebar picker (E5 S7)
   year_min: number | null;
   year_max: number | null;
   rating_max: number | null;