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/PlexBrowse.tsx

@@ -272,10 +272,15 @@ export default function PlexBrowse() {
   // (cast/crew) widens to the 'both' scope so their movies AND shows show up together (mixed feed).
   function applyFilter(patch: Partial<PlexFilters>) {
     const merged = { ...filters } as unknown as Record<string, unknown>;
+    const isPlainObject = (x: unknown): x is Record<string, unknown> =>
+      typeof x === "object" && x !== null && !Array.isArray(x);
     for (const [k, v] of Object.entries(patch)) {
       const cur = merged[k];
       if (Array.isArray(v) && Array.isArray(cur)) {
         merged[k] = Array.from(new Set([...(cur as unknown[]), ...(v as unknown[])]));
+      } else if (isPlainObject(v) && isPlainObject(cur)) {
+        // Merge label maps (collectionTitles / playlistTitles) so adding one keeps the others' titles.
+        merged[k] = { ...cur, ...v };
       } else {
         merged[k] = v;
       }