fix(library): podcasts list newest episode first

A Jellypod podcast listed its episodes alphabetically. The store pinned
SortBy=SortName onto every drill-down, which overrode the order the
channel plugin returns — and since Jellypod prefixes played episodes with
"[Played]", the name sort also clumped every heard episode at the top.

Which order a container's children take is domain knowledge, so it moves
to Rust: the caller names the container (GetItemsOptions.parentKind) and
default_listing_sort answers with the sort. A channel folder is
PremiereDate descending, every other container keeps SortName ascending,
and a caller naming no container still gets no SortBy, so the paths that
rely on the server's own order keep it. An explicit sort always wins.

ChannelFolderItem with is_folder now maps to MediaKind::ChannelFolder
instead of collapsing into Folder — while both were Folder there was
nothing to key the rule on. The offline leg of the cache/server race
applies the same order, so the cached list no longer flashes in name
order before the server's arrives.

TRACES: UR-007 | DR-257 | UT-229, UT-230, UT-231

src-tauri/src/domain/media.rs

@@ -48,6 +48,14 @@ pub enum MediaKind {
     /// seekable, unlike `LiveChannel`. Distinct from `Channel` (the container)
     /// and from `Other` so the UI can route it to playback.
     ChannelItem,
+    /// A *container* inside a channel — a Jellyfin `ChannelFolderItem` that is
+    /// itself a folder, e.g. one podcast within a podcast channel. Distinct
+    /// from `Folder` because its children are plugin content with an order of
+    /// their own (newest episode first), which a folder's name order silently
+    /// overrode.
+    ///
+    /// TRACES: UR-007 | DR-257
+    ChannelFolder,
     /// A kind we do not model explicitly. Reached only for provider item types
     /// that map to nothing meaningful; consumers treat it like an opaque
     /// container. The mapping must be *total* — it never panics — so this is the