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/LibraryListView.svelte

@@ -19,7 +19,7 @@
   }
 
   function getImageTag(item: MediaItem | Library): string | undefined {
-    return "primaryImageTag" in item ? (item.primaryImageTag ?? undefined) : ("imageTag" in item ? (item.imageTag ?? undefined) : undefined);
+    return "imageId" in item ? (item.imageId ?? undefined) : ("imageTag" in item ? (item.imageTag ?? undefined) : undefined);
   }
 
   function getSubtitle(item: MediaItem | Library): string {