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/lib/stores/library.ts

@@ -3,7 +3,7 @@
 
 import { writable, derived } from "svelte/store";
 import { listen, type UnlistenFn } from "@tauri-apps/api/event";
-import type { Library, MediaItem, SearchResult, Genre } from "$lib/api/types";
+import type { Library, MediaItem, MediaKind, SearchResult, Genre } from "$lib/api/types";
 import type { SearchOptions } from "$lib/api/bindings";
 import type { SearchScope } from "$lib/utils/searchScope";
 import { auth } from "./auth";
@@ -113,9 +113,21 @@ function createLibraryStore() {
     }
   }
 
+  // What a container's children are ordered by is domain knowledge, so the
+  // store names the *container* and Rust answers with the sort (see
+  // `default_listing_sort`). A channel folder — one podcast inside a plugin
+  // channel — is read newest-episode-first; naming `SortName` here, as this did
+  // for every drill-down, threw that order away.
+  //
+  // TRACES: UR-007 | DR-257 | UT-231
   async function loadItems(
     parentId: string,
-    options: { startIndex?: number; limit?: number; genres?: string[] } = {},
+    options: {
+      startIndex?: number;
+      limit?: number;
+      genres?: string[];
+      parentKind?: MediaKind;
+    } = {},
   ) {
     update((s) => ({ ...s, loadingCount: s.loadingCount + 1, error: null }));
 
@@ -129,8 +141,7 @@ function createLibraryStore() {
         startIndex: options.startIndex ?? 0,
         limit: options.limit ?? 10000,
         fields: ["PrimaryImageAspectRatio", "Overview", "MediaStreams"],
-        sortBy: "SortName",
-        sortOrder: "Ascending",
+        parentKind: options.parentKind ?? "folder",
         genres: options.genres,
       });