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/EpisodeFocusView.svelte

@@ -77,8 +77,8 @@
     if (episode.backdropImageTags?.[0]) {
       return { itemId: episode.id, imageType: "Backdrop" as const, tag: episode.backdropImageTags[0] };
     }
-    if (episode.primaryImageTag) {
-      return { itemId: episode.id, imageType: "Primary" as const, tag: episode.primaryImageTag };
+    if (episode.imageId) {
+      return { itemId: episode.id, imageType: "Primary" as const, tag: episode.imageId };
     }
     if (series.backdropImageTags?.[0]) {
       return { itemId: series.id, imageType: "Backdrop" as const, tag: series.backdropImageTags[0] };
@@ -246,7 +246,7 @@
             <CachedImage
               itemId={ep.id}
               imageType="Primary"
-              tag={ep.primaryImageTag}
+              tag={ep.imageId}
               maxWidth={400}
               alt={ep.name}
               class="w-full h-full object-cover transition-transform {isCurrent ? '' : 'group-hover/card:scale-105'}"