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/library/+page.svelte

@@ -127,30 +127,27 @@
     // Prevent accidental taps during scrolling (Android)
     if (scrollGuard.isScrollActive()) return;
 
-    if ("type" in item) {
+    if ("kind" in item) {
       // It's a MediaItem
       const mediaItem = item as MediaItem;
-      // A ChannelFolderItem can be a folder (drill in) or a playable leaf.
-      if (mediaItem.type === "ChannelFolderItem" && !mediaItem.isFolder) {
+      // A playable channel leaf plays directly; a channel container drills in.
+      if (mediaItem.kind === "channelItem") {
         goto(`/player/${mediaItem.id}`);
         return;
       }
-      switch (mediaItem.type) {
-        case "Series":
-        case "Movie":
-        case "MusicAlbum":
-        case "MusicArtist":
-        case "Folder":
-        case "CollectionFolder":
-        case "Playlist":
-        case "Channel":
-        case "ChannelFolderItem":
+      switch (mediaItem.kind) {
+        case "series":
+        case "movie":
+        case "album":
+        case "artist":
+        case "folder":
+        case "playlist":
+        case "channel":
           // Navigate to detail view
           goto(`/library/${mediaItem.id}`);
           break;
-        case "Episode":
-        case "TvChannel":
-          // Episodes and live TV channels play directly
+        case "episode":
+          // Episodes play directly
           goto(`/player/${mediaItem.id}`);
           break;
         default: