feat(plex): make the Collection filter multi-select with Any/All

A title can belong to several Plex collections, so the collection filter becomes
multi-select combining with Any (OR, default) / All (AND) — the same shape as the
genre/actor filters.

Backend (query.py): `collection` (single) → `collections` CSV + `collection_mode`
on LibraryFilters; meta_filter_conds ORs/ANDs `collection_keys.contains` per id.
Frontend: PlexFilters `collection`/`collectionTitle` → `collections` + optional
`collectionMode` + `collectionTitles` map; CollectionPicker is now a multi-select
Combobox with the any/all match footer; one chip per collection (usePlexActive-
Filters); the info-page "Browse collection" strip unions into the list; applyFilter
now shallow-merges label maps so adding one keeps the others' titles. PlexProvider
migrates a legacy persisted single `collection` into the new shape on read.

Test: test_plex_collections_filter (single / any-union / all-intersection).

frontend/src/components/PlexProvider.tsx

@@ -123,9 +123,20 @@ export function PlexProvider({ children }: { children: ReactNode }) {
   const filters = useMemo<PlexFilters>(() => {
     try {
       // Drop the legacy `sortDir` — direction now lives in the sort SortState. Left here it would leak
-      // into the facets query key and be re-persisted on every filter change.
-      const { sortDir: _drop, ...rest } = JSON.parse(filtersRaw);
-      return { ...EMPTY_PLEX_FILTERS, ...rest };
+      // into the facets query key and be re-persisted on every filter change. Also migrate the legacy
+      // single `collection`/`collectionTitle` into the multi-select `collections`/`collectionTitles`.
+      const {
+        sortDir: _drop,
+        collection: legacyCollection,
+        collectionTitle: legacyTitle,
+        ...rest
+      } = JSON.parse(filtersRaw);
+      const merged: PlexFilters = { ...EMPTY_PLEX_FILTERS, ...rest };
+      if (legacyCollection && !merged.collections.length) {
+        merged.collections = [legacyCollection];
+        if (legacyTitle) merged.collectionTitles = { [legacyCollection]: legacyTitle };
+      }
+      return merged;
     } catch {
       return EMPTY_PLEX_FILTERS;
     }