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/player/mod.rs

@@ -2177,6 +2177,7 @@ mod tests {
             id: id.to_string(),
             name: format!("Episode {}", index),
             item_type: "Episode".to_string(),
+            kind: crate::domain::MediaKind::Episode,
             is_folder: false,
             server_id: "server".to_string(),
             parent_id: Some("season1".to_string()),
@@ -2184,11 +2185,13 @@ 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,