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/GenericGenreBrowser.svelte

@@ -234,7 +234,7 @@
                 <CachedImage
                   itemId={item.id}
                   imageType="Primary"
-                  tag={item.primaryImageTag}
+                  tag={item.imageId}
                   maxWidth={300}
                   alt={item.name}
                   class="w-full h-full object-cover group-hover:scale-105 transition-transform"