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/domain/mod.rs

@@ -0,0 +1,10 @@
+//! Canonical, provider-neutral domain model — the single source of truth for
+//! the app's core data shapes, shared with the frontend via generated bindings.
+//!
+//! Spec: docs/specs/frontend-domain-model.md
+
+pub mod from_jellyfin;
+pub mod media;
+
+pub use from_jellyfin::{kind_from_jellyfin, ticks_to_ms};
+pub use media::MediaKind;