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/api/bindings.ts

@@ -1918,7 +1918,11 @@ export type MediaType = "audio" | "video"
  * Lightweight media item for merged playback state
  * Converts from both local MediaItem and remote NowPlayingItem
  */
-export type MergedMediaItem = { id: string; title: string; artist: string | null; album: string | null; albumId: string | null; duration: number | null; primaryImageTag: string | null; mediaType: string }
+export type MergedMediaItem = { id: string; title: string; artist: string | null; album: string | null; albumId: string | null; duration: number | null; primaryImageTag: string | null; 
+/**
+ * Neutral image identifier — replaces `primary_image_tag` (same value).
+ */
+imageId: string | null; mediaType: string }
 /**
  * Argument struct for [`set_network_state`].
  * 
@@ -2110,9 +2114,17 @@ artistItems?: ArtistItem[] | null;
  */
 artists?: string[] | null; 
 /**
- * Primary image tag for artwork
+ * Primary image tag for artwork.
+ * 
+ * Legacy Jellyfin name; being replaced by `image_id` (same value). Dual-carried
+ * while the frontend migrates (docs/specs/frontend-domain-model.md).
  */
 primaryImageTag?: string | null; 
+/**
+ * Neutral image identifier the frontend resolves to a URL — replaces
+ * `primary_image_tag`.
+ */
+imageId?: string | null; 
 /**
  * Item type (Audio, Movie, Episode, etc.)
  */