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/MediaCard.svelte

@@ -113,7 +113,7 @@
   });
 
   const imageTag = $derived(
-    "primaryImageTag" in item ? item.primaryImageTag : ("imageTag" in item ? item.imageTag : undefined)
+    "imageId" in item ? item.imageId : ("imageTag" in item ? item.imageTag : undefined)
   );
 
   const maxWidth = $derived(size === "large" ? 400 : size === "medium" ? 300 : 200);