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/hybrid.rs

@@ -1422,6 +1422,7 @@ mod tests {
             id: id.to_string(),
             name: name.to_string(),
             item_type: "Movie".to_string(),
+            kind: crate::domain::MediaKind::Movie,
             is_folder: false,
             server_id: "test-server".to_string(),
             parent_id: Some("parent-123".to_string()),
@@ -1429,11 +1430,13 @@ mod tests {
             overview: Some("Test overview".to_string()),
             genres: Some(vec!["Action".to_string(), "Adventure".to_string()]),
             runtime_ticks: Some(7200000000),
+            duration_ms: Some(720000),
             production_year: Some(2024),
             premiere_date: None,
             community_rating: Some(8.5),
             official_rating: Some("PG-13".to_string()),
             primary_image_tag: Some("image-tag-123".to_string()),
+            image_id: Some("image-tag-123".to_string()),
             backdrop_image_tags: Some(vec!["backdrop-1".to_string()]),
             parent_backdrop_image_tags: None,
             album_id: None,