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/i18n/locales/en/common.json

@@ -22,5 +22,8 @@
   "loadError": "Couldn't load this — check your connection and try again.",
   "seeAll": "See all",
   "scrollLeft": "Scroll left",
-  "scrollRight": "Scroll right"
+  "scrollRight": "Scroll right",
+  "search": "Search",
+  "remove": "Remove",
+  "noResults": "No matches"
 }

frontend/src/i18n/locales/en/plex.json

@@ -70,7 +70,10 @@
     "collection": "Collection",
     "collectionSearch": "Search collections…",
     "collectionEmpty": "No collections",
-    "matchLabel": "Match"
+    "matchLabel": "Match",
+    "studio": "Studio",
+    "add": "Add",
+    "moreRefine": "{{count}} more — refine with search"
   },
   "count": "{{count}} titles",
   "searchCount": "{{count}} matches",

frontend/src/i18n/locales/hu/common.json

@@ -22,5 +22,8 @@
   "loadError": "Nem sikerült betölteni — ellenőrizd a kapcsolatot, és próbáld újra.",
   "seeAll": "Összes",
   "scrollLeft": "Görgetés balra",
-  "scrollRight": "Görgetés jobbra"
+  "scrollRight": "Görgetés jobbra",
+  "search": "Keresés",
+  "remove": "Eltávolítás",
+  "noResults": "Nincs találat"
 }

frontend/src/i18n/locales/hu/plex.json

@@ -70,7 +70,10 @@
     "collection": "Kollekció",
     "collectionSearch": "Kollekciók keresése…",
     "collectionEmpty": "Nincs kollekció",
-    "matchLabel": "Egyezés"
+    "matchLabel": "Egyezés",
+    "studio": "Stúdió",
+    "add": "Hozzáadás",
+    "moreRefine": "még {{count}} — finomíts kereséssel"
   },
   "count": "{{count}} cím",
   "searchCount": "{{count}} találat",