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/routes/+page.svelte

@@ -57,14 +57,13 @@
   });
 
   function handleItemClick(item: MediaItem) {
-    switch (item.type) {
-      case "Series":
-      case "Season":
-      case "MusicAlbum":
-      case "MusicArtist":
-      case "Folder":
-      case "Channel":
-      case "ChannelFolderItem":
+    switch (item.kind) {
+      case "series":
+      case "season":
+      case "album":
+      case "artist":
+      case "folder":
+      case "channel":
         goto(`/library/${item.id}`);
         break;
       default:
@@ -100,7 +99,7 @@
 
   const heroItems = $derived($home.heroItems);
   const resumeItems = $derived($home.resumeItems.filter(
-    i => i.type === "Movie" || i.type === "Episode"
+    i => i.kind === "movie" || i.kind === "episode"
   ));
   const nextUpItems = $derived($home.nextUpItems);
   const latestItems = $derived($home.latestItems);