domain: primaryImageTag -> imageId end-to-end (phase 4a/4b)

Rust: PlayerMediaItem and MergedMediaItem gain image_id (dual-carry),
populated from primary_image_tag at every construction/conversion site.
Regenerated bindings.

Frontend: all catalog + player + merged readers now use imageId. The
NowPlayingItem->MediaItem bridge (player.ts) properly maps the remote
session's Jellyfin fields (Type, runTimeTicks, primaryImageTag) onto the
neutral kind/durationMs/imageId. Types that are genuinely out of scope
(Person, NowPlayingItem, PlayItemRequest) keep primaryImageTag.

Rust 456, frontend 644, check clean.

src/lib/components/library/PlaylistDetailView.svelte

@@ -146,11 +146,11 @@
   <div class="flex gap-6 pt-4">
     <!-- Playlist artwork -->
     <div class="flex-shrink-0 w-48">
-      {#if playlist.primaryImageTag}
+      {#if playlist.imageId}
         <CachedImage
           itemId={playlist.id}
           imageType="Primary"
-          tag={playlist.primaryImageTag}
+          tag={playlist.imageId}
           maxWidth={400}
           alt={playlist.name}
           class="w-full rounded-lg shadow-lg"