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/home/HeroBanner.svelte

@@ -32,7 +32,7 @@
     }
 
     // 2. For episodes, try series/season backdrops
-    if (currentItem.type === "Episode") {
+    if (currentItem.kind === "episode") {
       if (currentItem.seriesId && currentItem.parentBackdropImageTags?.[0]) {
         return { itemId: currentItem.seriesId, imageType: "Backdrop" as const, tag: currentItem.parentBackdropImageTags[0] };
       }
@@ -45,17 +45,17 @@
     }
 
     // 3. For music tracks, try album backdrop
-    if (currentItem.type === "Audio" && currentItem.albumId) {
+    if (currentItem.kind === "track" && currentItem.albumId) {
       return { itemId: currentItem.albumId, imageType: "Backdrop" as const, tag: undefined };
     }
 
     // 4. Fall back to primary image
-    if (currentItem.primaryImageTag) {
-      return { itemId: currentItem.id, imageType: "Primary" as const, tag: currentItem.primaryImageTag };
+    if (currentItem.imageId) {
+      return { itemId: currentItem.id, imageType: "Primary" as const, tag: currentItem.imageId };
     }
 
     // 5. Last resort for audio: album primary
-    if (currentItem.type === "Audio" && currentItem.albumId) {
+    if (currentItem.kind === "track" && currentItem.albumId) {
       return { itemId: currentItem.albumId, imageType: "Primary" as const, tag: undefined };
     }
 
@@ -190,7 +190,7 @@
             onclick={() => {
               // Navigate to full series detail page with cast/crew/related content
               // (even for episodes, show the series page so users see cast and related items)
-              if (currentItem.type === "Episode" && currentItem.seriesId) {
+              if (currentItem.kind === "episode" && currentItem.seriesId) {
                 goto(`/library/${currentItem.seriesId}`);
               } else {
                 goto(`/library/${currentItem.id}`);