domain: flip catalog frontend off Jellyfin item-type strings (phase 2a)

Migrate catalog MediaItem consumers from stringly item.type ("Audio",
"MusicAlbum", …) to the neutral item.kind enum across all classification
logic: home, library detail, player routing, artist/person/related/genre
components, tv store.

Model refinements found during migration (each a real distinction the
flat item_type collapsed):
- MediaKind::LiveChannel — live TV (playable, non-seekable) vs
- MediaKind::ChannelItem — channel VOD leaf (playable, seekable) vs
- MediaKind::Channel — channel container (drill-in).
  TvChannel->LiveChannel, non-folder ChannelFolderItem->ChannelItem.

RelatedItemsSection and GenreTags props migrated from Jellyfin type
strings to MediaKind; MediaKind re-exported from api/types.

Deferred by design: display {item.type} text, ResultsCounter labels,
Person.type (role), stream.type (phase 4), and all runTimeTicks/tick math
(coupled to playbackPositionTicks — phase 3). Old fields still dual-carried
so nothing breaks.

Rust 456 + 7 domain tests, frontend 644 tests, check clean.

src/lib/components/library/MediaCard.svelte

@@ -101,11 +101,11 @@
   };
 
   const isMusicType = $derived(
-    "type" in item && (item.type === "Audio" || item.type === "MusicAlbum" || item.type === "MusicArtist" || item.type === "Playlist")
+    "kind" in item && (item.kind === "track" || item.kind === "album" || item.kind === "artist" || item.kind === "playlist")
   );
 
   const aspectRatio = $derived(() => {
-    if ("type" in item) {
+    if ("kind" in item) {
       return isMusicType ? "aspect-square" : "aspect-[2/3]";
     }
     // Library