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-tauri/src/domain/media.rs

@@ -36,8 +36,18 @@ pub enum MediaKind {
     // Cast/crew
     Person,
     // Containers / live TV
+    /// A channel *container* the user drills into (Jellyfin `Channel`).
     Channel,
     Folder,
+    /// A live TV channel — playable, but a live stream with no seekable
+    /// timeline (no resume/seek). Jellyfin `TvChannel`/`LiveTvChannel`.
+    LiveChannel,
+    /// A playable leaf inside a channel (Jellyfin `ChannelFolderItem` that is
+    /// not itself a folder) — e.g. a plugin-channel VOD item that has no
+    /// dedicated item type but carries its own media streams. Playable and
+    /// seekable, unlike `LiveChannel`. Distinct from `Channel` (the container)
+    /// and from `Other` so the UI can route it to playback.
+    ChannelItem,
     /// A kind we do not model explicitly. Reached only for provider item types
     /// that map to nothing meaningful; consumers treat it like an opaque
     /// container. The mapping must be *total* — it never panics — so this is the