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/AudioPlayer.svelte

@@ -143,7 +143,7 @@
         <CachedImage
           itemId={artworkItemId}
           imageType="Primary"
-          tag={displayMedia?.primaryImageTag}
+          tag={displayMedia?.imageId}
           maxWidth={800}
           alt=""
           class="w-full h-full object-cover blur-3xl opacity-30"
@@ -223,7 +223,7 @@
           <CachedImage
             itemId={artworkItemId}
             imageType="Primary"
-            tag={displayMedia?.primaryImageTag}
+            tag={displayMedia?.imageId}
             maxWidth={500}
             alt={displayMedia?.name}
             class="w-full h-full object-cover"