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/online.rs

@@ -619,10 +619,13 @@ impl JellyfinItem {
         let primary_tag = self.image_tags.as_ref().and_then(|tags| tags.primary());
         let backdrop_tags = self.backdrop_image_tags;
 
+        let kind = crate::domain::kind_from_jellyfin(&self.item_type, self.is_folder);
+
         MediaItem {
             id: self.id,
             name: self.name,
             item_type: self.item_type,
+            kind,
             is_folder: self.is_folder,
             server_id,
             parent_id: self.parent_id,
@@ -634,7 +637,9 @@ impl JellyfinItem {
             community_rating: self.community_rating,
             official_rating: self.official_rating,
             runtime_ticks: self.run_time_ticks,
-            primary_image_tag: primary_tag,
+            duration_ms: self.run_time_ticks.map(crate::domain::ticks_to_ms),
+            primary_image_tag: primary_tag.clone(),
+            image_id: primary_tag,
             backdrop_image_tags: backdrop_tags,
             parent_backdrop_image_tags: self.parent_backdrop_image_tags,
             album_id: self.album_id,
@@ -923,6 +928,7 @@ impl MediaRepository for OnlineRepository {
                         .clone()
                         .unwrap_or_else(|| "Unknown Album".to_string()),
                     item_type: "MusicAlbum".to_string(),
+                    kind: crate::domain::MediaKind::Album,
                     is_folder: true,
                     server_id: first_track.server_id.clone(),
                     parent_id: None,
@@ -934,7 +940,9 @@ impl MediaRepository for OnlineRepository {
                     community_rating: None,
                     official_rating: None,
                     runtime_ticks: None,
+                    duration_ms: None,
                     primary_image_tag: first_track.primary_image_tag.clone(),
+                    image_id: first_track.primary_image_tag.clone(),
                     backdrop_image_tags: None,
                     parent_backdrop_image_tags: None,
                     album_id: None,