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/routes/library/[id]/+page.svelte

@@ -355,11 +355,11 @@
       <div class="flex gap-6">
         <!-- Poster -->
         <div class="flex-shrink-0 w-48">
-          {#if item.primaryImageTag}
+          {#if item.imageId}
             <CachedImage
               itemId={item.id}
               imageType="Primary"
-              tag={item.primaryImageTag}
+              tag={item.imageId}
               maxWidth={400}
               alt={item.name}
               class="w-full {isMusicItem ? 'aspect-square' : ''} rounded-lg shadow-lg"

src/routes/player/[id]/+page.svelte

@@ -355,8 +355,8 @@
                   artist: t.artists?.join(", ") || null,
                   album: t.albumName || null,
                   duration: t.runTimeTicks ? t.runTimeTicks / 10000000 : null,
-                  artworkUrl: t.primaryImageTag
-                    ? repo.getImageUrl(t.albumId || t.id, "Primary", { maxWidth: 300, tag: t.primaryImageTag })
+                  artworkUrl: t.imageId
+                    ? repo.getImageUrl(t.albumId || t.id, "Primary", { maxWidth: 300, tag: t.imageId })
                     : null,
                   mediaType: "audio",
                   streamUrl: trackStreamUrl,