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/stores/movies.ts

@@ -45,7 +45,7 @@ function createMoviesStore() {
 
   /** Artwork check for hero candidates: needs a backdrop or a primary image. */
   const hasArt = (i: MediaItem) =>
-    !!(i.backdropImageTags && i.backdropImageTags.length > 0) || !!i.primaryImageTag;
+    !!(i.backdropImageTags && i.backdropImageTags.length > 0) || !!i.imageId;
 
   async function loadSections(libraryId: string) {
     update(s => ({