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/ArtistDetailView.svelte

@@ -112,12 +112,12 @@
     <!-- Artist Info -->
     <div class="flex flex-col items-center text-center py-12">
       <!-- Artist Image -->
-      {#if artist.primaryImageTag}
+      {#if artist.imageId}
         <div class="mb-6 rounded-full overflow-hidden w-40 h-40 shadow-lg">
           <CachedImage
             itemId={artist.id}
             imageType="Primary"
-            tag={artist.primaryImageTag}
+            tag={artist.imageId}
             maxWidth={400}
             alt={artist.name}
             class="w-full h-full object-cover"
@@ -160,11 +160,11 @@
             class="group cursor-pointer"
           >
             <div class="aspect-square bg-[var(--color-surface)] rounded-lg overflow-hidden mb-2 group-hover:opacity-80 transition-opacity">
-              {#if album.primaryImageTag}
+              {#if album.imageId}
                 <CachedImage
                   itemId={album.id}
                   imageType="Primary"
-                  tag={album.primaryImageTag}
+                  tag={album.imageId}
                   maxWidth={200}
                   alt={album.name}
                   class="w-full h-full object-cover"
@@ -218,11 +218,11 @@
               class="group text-center"
             >
               <div class="w-32 h-32 bg-[var(--color-surface)] rounded-full overflow-hidden mb-2 mx-auto group-hover:opacity-80 transition-opacity">
-                {#if relatedArtist.primaryImageTag}
+                {#if relatedArtist.imageId}
                   <CachedImage
                     itemId={relatedArtist.id}
                     imageType="Primary"
-                    tag={relatedArtist.primaryImageTag}
+                    tag={relatedArtist.imageId}
                     maxWidth={200}
                     alt={relatedArtist.name}
                     class="w-full h-full object-cover"