feat(plex): restore Collections filter as a cross-library union picker

The searchable Collections picker was dropped from the sidebar when browsing
went unified (movies+shows across libraries) because collections are per-library
and there's no single selected library. Bring it back library-agnostic: GET
/api/plex/collections `library` is now optional — without it the endpoint unions
collections across all enabled libraries (with it, the old single-library path the
collection editor uses). The sidebar re-adds the searchable list (chip when one is
active). Its query key is ["plex-collections","union",<search>] so a search term
equal to a library plex_key can't collide with the editor's ["plex-collections",<key>],
while the shared prefix keeps editor invalidation refreshing the picker.

backend/app/routes/plex.py

@@ -748,16 +748,25 @@ def _collection_card(c: PlexCollection) -> dict:
 
 @router.get("/collections")
 def collections(
-    library: str,
+    library: str | None = None,
     q: str | None = None,
     user: User = Depends(current_user),
     db: Session = Depends(get_db),
 ) -> dict:
-    """The library's mirrored collections as cards (most-populated first). Optional name filter `q`."""
-    lib = db.query(PlexLibrary).filter_by(plex_key=str(library)).first()
-    if lib is None:
-        return {"collections": []}
-    query = db.query(PlexCollection).filter(PlexCollection.library_id == lib.id)
+    """Mirrored collections as cards (most-populated first), optionally name-filtered by `q`. With a
+    `library` plex_key → that one library's collections (used by the collection editor). WITHOUT one →
+    the UNION across all enabled libraries (the unified-scope sidebar picker, which has no single
+    library selected)."""
+    if library is not None:
+        lib = db.query(PlexLibrary).filter_by(plex_key=str(library)).first()
+        if lib is None:
+            return {"collections": []}
+        query = db.query(PlexCollection).filter(PlexCollection.library_id == lib.id)
+    else:
+        enabled_ids = [lb.id for lb in db.query(PlexLibrary.id).filter_by(enabled=True)]
+        if not enabled_ids:
+            return {"collections": []}
+        query = db.query(PlexCollection).filter(PlexCollection.library_id.in_(enabled_ids))
     term = (q or "").strip()
     if term:
         query = query.filter(PlexCollection.title.ilike(f"%{_like_escape(term)}%"))