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/stores/tv.ts

@@ -81,7 +81,7 @@ function createTvStore() {
 
       // Resume items are already video-only from the server, but keep episodes
       // (and the occasional movie that lives in a mixed library) defensively.
-      const continueWatching = resume.filter(i => i.type === "Episode" || i.type === "Movie");
+      const continueWatching = resume.filter(i => i.kind === "episode" || i.kind === "movie");
 
       // Mix the hero: in-progress episodes first (most personal), then next-up,
       // recent additions, and random series from across the library.