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/player/MiniPlayer.svelte

@@ -326,7 +326,7 @@
             <CachedImage
               itemId={displayMedia.albumId || displayMedia.id}
               imageType="Primary"
-              tag={displayMedia.primaryImageTag}
+              tag={displayMedia.imageId}
               maxWidth={100}
               alt={displayMedia?.name}
               class="w-full h-full object-cover"