domain: neutral StreamKind for media streams (phase 4d)

Add StreamKind enum (audio/video/subtitle/other) to the domain module with
a total stream_kind_from_jellyfin mapper. MediaStream gains a kind field
(dual-carry), populated at the mapping seam. Frontend VideoPlayer track/
subtitle selection and the channel-video check now use stream.kind instead
of the Jellyfin stream.type string.

Rust 456 (+ stream_kinds_map test), frontend 644, check clean.

src-tauri/src/domain/mod.rs

@@ -6,5 +6,5 @@
 pub mod from_jellyfin;
 pub mod media;
 
-pub use from_jellyfin::{kind_from_jellyfin, ticks_to_ms};
-pub use media::MediaKind;
+pub use from_jellyfin::{kind_from_jellyfin, stream_kind_from_jellyfin, ticks_to_ms};
+pub use media::{MediaKind, StreamKind};