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/from_jellyfin.rs

@@ -72,7 +72,7 @@ pub fn kind_from_jellyfin(item_type: &str, is_folder: bool) -> MediaKind {
         // channel leaf (distinct kind so the UI can route it to playback).
         "ChannelFolderItem" => {
             if is_folder {
-                MediaKind::Folder
+                MediaKind::ChannelFolder
             } else {
                 MediaKind::ChannelItem
             }
@@ -142,11 +142,18 @@ mod tests {
         assert_eq!(kind_from_jellyfin("BoxSet", true), MediaKind::Folder);
     }
 
+    /// A channel container is not an ordinary folder. Jellyfin gives both the
+    /// same item type, but only the channel one holds plugin content whose
+    /// natural order is by release date — a podcast, for instance. Collapsing
+    /// it into `Folder` left the repository with no way to tell the two apart,
+    /// so every podcast listed alphabetically.
+    ///
+    /// TRACES: UR-007 | DR-257 | UT-230
     #[test]
     fn channel_folder_item_disambiguates_on_is_folder() {
         assert_eq!(
             kind_from_jellyfin("ChannelFolderItem", true),
-            MediaKind::Folder
+            MediaKind::ChannelFolder
         );
         assert_eq!(
             kind_from_jellyfin("ChannelFolderItem", false),