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/backend.rs

@@ -380,6 +380,7 @@ 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),
@@ -438,6 +439,7 @@ 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),
@@ -490,6 +492,7 @@ 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),

src-tauri/src/player/media.rs

@@ -67,9 +67,16 @@ pub struct MediaItem {
     /// Artists as array of strings (fallback when artist_items not available)
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub artists: Option<Vec<String>>,
-    /// 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).
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub primary_image_tag: Option<String>,
+    /// Neutral image identifier the frontend resolves to a URL — replaces
+    /// `primary_image_tag`.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub image_id: Option<String>,
     /// Item type (Audio, Movie, Episode, etc.)
     #[serde(rename = "type", default, skip_serializing_if = "Option::is_none")]
     pub item_type: Option<String>,
@@ -345,6 +352,7 @@ mod tests {
             artist_items: None,
             artists: None,
             primary_image_tag: None,
+            image_id: None,
             item_type: None,
             playlist_id: None,
             duration: None,
@@ -380,6 +388,7 @@ mod tests {
             artist_items: None,
             artists: None,
             primary_image_tag: None,
+            image_id: None,
             item_type: None,
             playlist_id: None,
             duration: None,
@@ -414,6 +423,7 @@ mod tests {
             artist_items: None,
             artists: None,
             primary_image_tag: None,
+            image_id: None,
             item_type: None,
             playlist_id: None,
             duration: None,
@@ -448,6 +458,7 @@ mod tests {
             artist_items: None,
             artists: None,
             primary_image_tag: None,
+            image_id: None,
             item_type: None,
             playlist_id: None,
             duration: None,
@@ -489,6 +500,7 @@ mod tests {
             artist_items: None,
             artists: None,
             primary_image_tag: None,
+            image_id: None,
             item_type: None,
             playlist_id: None,
             duration: None,
@@ -523,6 +535,7 @@ 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(120.0),

src-tauri/src/player/mod.rs

@@ -1397,6 +1397,7 @@ 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),

src-tauri/src/player/queue.rs

@@ -551,6 +551,7 @@ mod tests {
                 artist_items: None,
                 artists: Some(vec!["Artist".to_string()]),
                 primary_image_tag: None,
+                image_id: None,
                 item_type: Some("Audio".to_string()),
                 playlist_id: None,
                 duration: Some(180.0),

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),

src-tauri/src/player/state.rs

@@ -323,6 +323,7 @@ mod tests {
             artist_items: None,
             artists: None,
             primary_image_tag: None,
+            image_id: None,
             item_type: Some("Video".to_string()),
             playlist_id: None,
             duration: Some(100.0),