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-tauri/src/player/session.rs

@@ -242,6 +242,7 @@ mod tests {
             artist_items: None,
             artists: Some(vec!["Test Artist".to_string()]),
             primary_image_tag: None,
+            image_id: None,
             item_type: Some("Audio".to_string()),
             playlist_id: None,
             duration: Some(180.0),
@@ -272,6 +273,7 @@ mod tests {
             artist_items: None,
             artists: None,
             primary_image_tag: None,
+            image_id: None,
             item_type: Some("Movie".to_string()),
             playlist_id: None,
             duration: Some(7200.0),