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/ArtistDetailView.svelte

@@ -43,7 +43,7 @@
           sortBy: "DateCreated",
           sortOrder: "Descending"
         });
-        albums = albumsResult.items.filter(item => item.type === "MusicAlbum");
+        albums = albumsResult.items.filter(item => item.kind === "album");
       } catch (e) {
         console.warn("Failed to load albums:", e);
       } finally {
@@ -58,7 +58,7 @@
           sortBy: "CommunityRating",
           sortOrder: "Descending"
         });
-        topTracks = tracksResult.items.filter(item => item.type === "Audio");
+        topTracks = tracksResult.items.filter(item => item.kind === "track");
       } catch (e) {
         console.warn("Failed to load tracks:", e);
       } finally {
@@ -76,7 +76,7 @@
             sortOrder: "Descending"
           });
           relatedArtists = relatedResult.items
-            .filter(item => item.id !== artist.id && item.type === "MusicArtist")
+            .filter(item => item.id !== artist.id && item.kind === "artist")
             .slice(0, 6);
         }
       } catch (e) {