domain: introduce provider-neutral media model (phase 1)

Establish src-tauri/src/domain/ as the single source of truth for the
media model, with all Jellyfin translation isolated in from_jellyfin.rs.
Adds MediaKind enum and neutral duration_ms/image_id fields to MediaItem
as additive, defaulted dual-carry alongside the legacy Jellyfin-named
fields, so nothing breaks while the frontend migrates off them.

- domain/media.rs: canonical MediaKind (closed enum, replaces stringly
  item_type), Default = Other so unknown/defaulted items are inert.
- domain/from_jellyfin.rs: total, panic-free item_type -> MediaKind
  classification (all audited types + person subroles) and ticks->ms.
- MediaItem gains kind/duration_ms/image_id, populated at both mapping
  seams (online to_media_item, offline cached_item_to_media_item) and
  the synthesized-album/person sites.
- Regenerated bindings.ts: frontend now HAS the neutral model available.

Phase 1 of docs/specs/frontend-domain-model.md. No frontend behaviour
change yet; wire shape is a superset of before.

Rust 456 tests, frontend 644 tests, check + check:boundary all green.

src-tauri/src/repository/offline.rs

@@ -68,10 +68,13 @@ impl OfflineRepository {
             .and_then(|s| serde_json::from_str::<Vec<String>>(s).ok())
             .unwrap_or_default();
 
+        let kind = crate::domain::kind_from_jellyfin(&item.item_type, item.is_folder);
+
         MediaItem {
             id: item.id.clone(),
             name: item.name,
             item_type: item.item_type,
+            kind,
             is_folder: item.is_folder,
             server_id: item.server_id,
             parent_id: item.parent_id,
@@ -82,11 +85,13 @@ impl OfflineRepository {
                 .as_ref()
                 .and_then(|s| serde_json::from_str::<Vec<String>>(s).ok()),
             runtime_ticks: item.runtime_ticks,
+            duration_ms: item.runtime_ticks.map(crate::domain::ticks_to_ms),
             production_year: item.production_year,
             premiere_date: item.premiere_date,
             community_rating: item.community_rating,
             official_rating: item.official_rating,
-            primary_image_tag: item.primary_image_tag,
+            primary_image_tag: item.primary_image_tag.clone(),
+            image_id: item.primary_image_tag,
             backdrop_image_tags: item.backdrop_image_tags,
             parent_backdrop_image_tags: item.parent_backdrop_image_tags,
             album_id: item.album_id,
@@ -1599,6 +1604,7 @@ impl MediaRepository for OfflineRepository {
             id: person_data.0,
             name: person_data.1,
             item_type: "Person".to_string(),
+            kind: crate::domain::MediaKind::Person,
             is_folder: false,
             server_id: self.server_id.clone(),
             parent_id: None,
@@ -1606,11 +1612,13 @@ impl MediaRepository for OfflineRepository {
             overview: person_data.2,
             genres: None,
             runtime_ticks: None,
+            duration_ms: None,
             production_year: None,
             premiere_date: None,
             community_rating: None,
             official_rating: None,
-            primary_image_tag: person_data.3,
+            primary_image_tag: person_data.3.clone(),
+            image_id: person_data.3,
             backdrop_image_tags: None,
             parent_backdrop_image_tags: None,
             album_id: None,
@@ -2100,6 +2108,7 @@ mod tests {
             id: id.to_string(),
             name: name.to_string(),
             item_type: "Audio".to_string(),
+            kind: crate::domain::MediaKind::Track,
             is_folder: false,
             server_id: "test-server".to_string(),
             parent_id: parent_id.map(|s| s.to_string()),
@@ -2107,11 +2116,13 @@ mod tests {
             overview: None,
             genres: None,
             runtime_ticks: None,
+            duration_ms: None,
             production_year: None,
             premiere_date: None,
             community_rating: None,
             official_rating: None,
             primary_image_tag: None,
+            image_id: None,
             backdrop_image_tags: None,
             parent_backdrop_image_tags: None,
             album_id: None,