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/playlist/AddToPlaylistModal.svelte

@@ -121,11 +121,11 @@
               class="w-full flex items-center gap-3 p-3 hover:bg-[var(--color-surface-hover)] rounded-lg transition-colors disabled:opacity-50"
             >
               <div class="w-10 h-10 flex-shrink-0 rounded overflow-hidden">
-                {#if playlist.primaryImageTag}
+                {#if playlist.imageId}
                   <CachedImage
                     itemId={playlist.id}
                     imageType="Primary"
-                    tag={playlist.primaryImageTag}
+                    tag={playlist.imageId}
                     maxWidth={80}
                     alt={playlist.name}
                     class="w-full h-full object-cover"