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).

backend/app/plex/query.py

@@ -142,8 +142,10 @@ def meta_filter_conds(model, p: dict, lang: str = "en") -> list:
         conds.append(model.studio.in_(stds))
     for kw in split_csv(p.get("keywords")):  # AND — TMDB keyword chips from the detail page (E5 S5)
         conds.append(model.keywords.contains([kw]))
-    if p.get("collection"):
-        conds.append(model.collection_keys.contains([p["collection"]]))
+    csel = split_csv(p.get("collections"))
+    if csel:  # OR (any) or AND (all) — a title can belong to several collections, mirroring genre_mode
+        cc = [model.collection_keys.contains([c]) for c in csel]
+        conds.append(and_(*cc) if p.get("collection_mode") == "all" else or_(*cc))
     return conds
 
 
@@ -174,7 +176,8 @@ class LibraryFilters:
     )
     studios: str | None = None
     keywords: str | None = None  # TMDB keyword chips (E5 S5), CSV; language-neutral
-    collection: str | None = None
+    collections: str | None = None  # CSV of collection rating_keys
+    collection_mode: str = "any"  # how multiple collections combine: any (OR, default) | all (AND)
 
     def as_dict(self) -> dict:
         return {
@@ -192,7 +195,8 @@ class LibraryFilters:
             "actor_mode": self.actor_mode,
             "studios": self.studios,
             "keywords": self.keywords,
-            "collection": self.collection,
+            "collections": self.collections,
+            "collection_mode": self.collection_mode,
         }